Pulsed ultrasound modulated optical tomography with increased optical/ultrasound pulse ratio

ABSTRACT

A system and method of performing ultrasound modulated optical tomography. Ultrasound is delivered into a target voxel in an anatomical structure, and sample light is delivered into the anatomical structure, whereby a portion of the sample light passing through the target voxel is scattered by the biological tissue as signal light, and a portion of the sample light not passing through the target voxel is scattered by the anatomical structure as background light. The ultrasound and sample light are pulsed in synchrony, such that only the signal light is frequency shifted by the ultrasound. Multiple pulses of the sample light are delivered into the anatomical structure for each pulse of the ultrasound delivered into the target voxel. Reference light is combined with the signal light and background light to generate an interference light pattern, which is sequentially modulated to generate different interference light patterns, which are detected.

RELATED APPLICATION DATA

Pursuant to 35 U.S.C. § 119(e), this application claims the benefit ofU.S. Provisional Patent Application 62/590,150, filed Nov. 22, 2017, andU.S. Provisional Patent Application 62/596,446, filed Dec. 8, 2017,which are expressly incorporated herein by reference. This applicationis also related to U.S. patent application Ser. No. 15/______ (AttorneyDocket No. KNL-001US01) and U.S. patent application Ser. No. 15/______(Attorney Docket No. KNL-001US03), filed on the same date, which areexpressly incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to methods and systems for non-invasivemeasurements in the human body, and in particular, methods and systemsrelated to detecting physiologically dependent optical parameters in thehuman body.

BACKGROUND OF THE INVENTION

Measuring neural activity in the brain is useful for medicaldiagnostics, neuromodulation therapies, neuroengineering, orbrain-computer interfacing. For example, it may be desirable to measureneural activity in the brain of a patient to determine if a particularregion of the brain has been impacted by reduced blood irrigation, ahemorrhage, any other type of damage. For instance, in cases where thepatient has suffered a traumatic brain injury, such as stroke, it may bedesirable to determine whether the patient should undergo a therapeuticprocedure. Measuring neural activity in the brain also may be used todetermine the efficacy of such a therapeutic procedure.

Conventional methods for measuring neural activity in the brain includediffuse optical tomography (DOT), and functional near-infraredspectroscopy (fNIRS), as well as others. These applications only employa moderate amount of near-infrared or visible light radiation, thusbeing comparatively safe and gentle for a biological subject incomparison to X-Ray Computed Tomography (CT) scans, positron emissiontomography (PET), or other methods that use higher-energy andpotentially harmful radiation. Moreover, in contrast to methods, such asfunctional magnetic resonance imaging (fMRI), these optically-basedimaging methods do not require large magnets or magnetic shielding, andthus, can be scaled to wearable or portable form factors, which isespecially important in applications such as brain-computer interfacing.

Because DOT and fNIRS rely on light, which scatters many times insidebrain, skull, dura, pia, and skin tissues, the light paths occurring inthese techniques comprise random or “diffusive” walks, and therefore,only limited spatial resolution can be obtained by a conventionaloptical detector, often on the order of centimeters. The reason for thislimited spatial resolution is that the paths of photons striking thedetector in such schemes are highly variable and difficult, and evenimpossible to predict without detailed microscopic knowledge of thescattering characteristics of the brain volume of interest, which istypically unavailable in practice (i.e., in the setting of non-invasivemeasurements through skull for brain imaging and brain interfacing). Insummary, light scattering prevents optical imaging from achieving highresolution deep inside tissue.

There is an increasing interest in ultrasound modulated opticaltomography (UOT) to detect more precisely localized changes inbiological tissues, e.g., on a sub-millimeter length scale, inside thickbiological tissue, such as the brain (see U.S. Pat. No. 8,423,116;Sakadzic S, Wang L V, “High-Resolution Ultrasound-Modulated OpticalTomography in Biological Tissues,” Optics Letters, Vol. 29, No. 23, pp.2770-2772, Dec. 1, 2004). These localized changes may include changes inlight absorption in the brain that reflect neural activity andneurovascular coupling, such as a blood-oxygen-level dependent signal,for application in diagnostics, therapeutics, or, notably, braincomputer interfacing (see Steinbrink J, Villringer A, Kempf F, Haux D.Boden S, Obrig H., “Illuminating the BOLD Signal: Combined fMRI-fNIRSStudies,” Magnetic Resonance Imaging, Vol. 24, No. 4, pp. 495-505, May31, 2006). Thus, there is an increasing interest in ultrasound modulatedoptical tomography (UOT) in biomedical applications due to its potentialto simultaneously achieve good resolution and imaging depth.

In UOT, a highly localized ultrasound focus, e.g., millimeter orsub-millimeter in size, is used to selectively perturb (i.e., “tag”)light (e.g., light generated by a near-infrared coherent laser) passingthrough a voxel size of tissue defined by the size of the ultrasoundfocus. Due to the acousto-optic effect, light passing through theultrasonic beam undergoes a frequency shift defined by multiples of theultrasonic frequency. By detecting the frequency-shifted light, i.e.,the tagged light, spatial information characterizing the biologicaltissue within the voxel can be acquired. As a result, spatial resolutionis boosted from the centimeter-scale diffusive spread of light in thebiological tissue to approximately a millimeter-scale voxel size. Thisultrasound tagging of light relies on mechanisms known in the field (seeMahan G D, Engler W E, Tiemann J J, Uzgiris E, “Ultrasonic Tagging ofLight: Theory,” Proceedings of the National Academy of Sciences, Vol.95, No. 24, pp. 14015-14019, Nov. 24, 1998).

Typical UOT implementations generate weak signals that make it difficultto differentiate ultrasound-tagged light passing through the focal voxelfrom a much larger amount of unmodulated light which is measured as DCshot noise. Thus, conventional UOT has the challenge of obtainingoptical information through several centimeters of biological tissue,for example, noninvasive measurements through the human skull used tomeasure functional changes in the brain.

Various methods have been developed to detect the very small fraction oftagged light out of a large background of untagged light by detectingthe speckle pattern of light resulting from the interference of manymultiply-scattered optical waves with different phases and amplitudes,which combine in a resultant wave whose amplitude, and thereforeintensity, as well as phase, varies randomly. In the context ofneuroengineering and brain computer interfacing, a key challenge is torender these methods to be sufficiently sensitive to be useful forthrough-human-skull functional neuroimaging.

One technique uses a narrow spectral filter to separate out the untaggedlight striking a single-pixel detector, and is immune to speckledecorrelation (greater than ˜0.1 ms-1 ms) due to the scatters' motion(for example, blood flow) inside living biological tissue, but requiresbulky and expensive equipment.

Another technique uses crystal-based holography to combine a referencelight beam and the sample light beam into a constructive interferencepattern, but can be adversely affected by rapid speckle decorrelation,since the response time of the crystal is usually much longer than thespeckle correlation time.

Still another technique, referred to as heterodyne parallel speckledetection (PSD), employs optical interference together with a spatiallyresolved detector array (e.g., a conventional charge-coupled device(CCD) camera) used as an array of independent detectors for collectingthe signal over a large number of coherence areas (see Atlan M, Forget BC, Ramaz F, Boccara A C, Gross M, “Pulsed Acousto-Optic Imaging inDynamic Scattering Media With Heterodyne Parallel Speckle Detection,”Optics Letter, Vol. 30, No. 11, pp. 1360-1362, Jun. 1, 2005). Suchconfiguration improves the signal-to-noise ratio relative to asingle-detector and relative to approaches based on other modes ofseparating tagged and untagged light, such as spectral filters. However,the conventional CCD cameras used for heterodyne PSD have low framerates, and therefore suffer from a relatively low speed relative to thespeckle decorrelation time, thereby making this set up insufficient forin vivo deep tissue applications. Furthermore, conventional CCD camerasrecord both the AC signal and the DC background for each pixel. Thus,only a few bits of a pixel value can be used to represent the useful ACsignal, while most of the bits are wasted in representing the DCbackground, resulting in a low efficiency in the use of bits.

Lock-in cameras, as compared to conventional CCD cameras, have been usedfor comparatively bit-efficient and noise resistant heterodyne PSD (seeLiu Y, Shen Y, Ma C, Shi J, Wang L V, “Lock-in Camera Based HeterodyneHolography for Ultrasound-Modulated Optical Tomography Inside DynamicScattering Media,” Applied Physics Letters, Vol. 108, No. 23, 231106,Jun. 6, 2016; see also Liu Y, Ma C, Shen Y, Wang L V, “Bit-Efficient,Sub-Millisecond Wavefront Measurement Using a Lock-In Camera forTime-Reversal Based Optical Focusing Inside Scattering Media,” OpticsLetters, Vol. 41, No. 7, pp. 1321-1324, Apr. 1, 2016). For each pixel, alock-in camera is capable of performing lock-in detection and outputtingonly information of the AC signal as a single AC amplitude map that istransferred to a computer, and thus, provides an efficient means ofdetecting and processing the speckle pattern.

Besides the challenges posed by the signal-to-noise ratio, speckledecorrelation time, and efficient pixel bit processing, anotherchallenge involves obtaining sufficient axial resolution (i.e., thedepth resolution or ultrasound propagation direction). To address thischallenge, UOT has been applied in a pulsed wave (PW) mode forheterodyne PSD, rather than a continuous (CW) mode (see Li Y Zhang H,Kim C, Wagner K H, Hemmer P., Wang L V, “Pulsed Ultrasound-ModulatedOptical Tomography Using Spectral-Hole Burning as a Narrowband SpectralFilter,” Applied Physics Letters, Vol. 93, No. 1, 011111, Jul. 7, 2008;Ruan H, Mather M L, Morgan S P, “Pulsed Ultrasound Modulated OpticalTomography with Harmonic Lock-In Holography Detection,” JOSA A, Vol. 30,No. 7, pp. 1409-1416, Jul. 1, 2013).

PW UOT has the benefit of enabling improved axial resolution compared toCW UOT. That is, with CW UOT, any light passing through the tissue, eventhough outside of the focal voxel, may be inadvertently tagged by thecontinuously propagating ultrasound energy along the ultrasound axis,thereby decreasing the signal-to-noise ratio. With PW UOT, the lightpassing through the tissue is pulsed only when the ultrasound pulsestravels through the focal voxel, such that light outside of the focalvoxel will not be tagged by the ultrasound energy. Although PW UOTimproves axial resolution, the pulsed UOT signals are weak relative tocontinuous UOT signals.

Although the UOT schemes described above may be sufficient for certainapplications, such UOT schemes are inappropriate for the application of3D-resolved, highly sensitive detection of small signals (e.g.,blood-oxygen-level dependent signals) non-invasively through thickscattering layers, such as the human skull.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present inventions, anultrasound modulated optical tomography (UOT) system comprises anacoustic assembly configured for delivering ultrasound into a targetvoxel (e.g., one comprising brain matter) within an anatomicalstructure. The target voxel may be relatively small, e.g., less than onemm³.

The UOT system further comprises an interferometer configured fordelivering sample light into the anatomical structure, whereby a portionof the sample light passing through the target voxel is scattered by theanatomical structure as signal light, and another portion of the samplelight not passing through the target voxel is scattered by theanatomical structure as background light that combines with the signallight to create a sample light pattern. The interferometer is furtherconfigured for combining reference light with the sample light patternto generate an interference light pattern. The reference light may becombined with the signal light in a homodyne manner. For example, theinterferometer may be further configured for frequency shifting thesample light by the frequency of the ultrasound, such that the referencelight is combined with the signal light in the homodyne manner. In oneembodiment, the interferometer comprises a light source configured forgenerating source light, a beam splitter configured for splitting thesource light into the sample light and the reference light, and a lightcombiner configured for combining the reference light with the signallight and the background light to generate the interference lightpattern.

The UOT system further comprises a controller configured for operatingthe acoustic assembly and the interferometer to pulse the ultrasound andthe sample light in synchrony, such that only the signal light isfrequency shifted by the ultrasound. Multiple pulses of the sample lightare delivered into the anatomical structure for each pulse of theultrasound delivered into the target voxel. In one embodiment, thepulses of the sample light are identical. In this case, theinterferometer may comprise at least one 1×N fiber splitter and at leastone N×1 fiber coupler configured for generating the identical pulses ofthe sample light from a single optical pulse.

The controller is further configured for operating the interferometer tosequentially modulate the interference light pattern to generate aplurality of different interference light patterns. In one embodiment,the interferometer is configured for sequentially modulating theinterference light pattern by phase modulating the interference lightpattern, e.g., by setting different phase differences (e.g., 0, π/2, π,and 3π/2) between sequential pulses of the sample light and thereference light. In this case, the interferometer may comprise anoptical phase shifter configured for setting a phase difference betweenthe sample light and the reference light to phase modulate theinterference light pattern.

In another embodiment, the controller is configured for operating theacoustic assembly and the interferometer to pulse the ultrasound and thesample light in synchrony, such that only a single pulse of the samplelight is delivered into the anatomical structure for each pulse of theultrasound delivered into the target voxel. In still another embodiment,the controller is configured for operating the acoustic assembly and theinterferometer to pulse the ultrasound and the sample light insynchrony, such that multiple pulses of the sample light are deliveredinto the anatomical structure for each pulse of the ultrasound deliveredinto the target voxel.

The UOT system further comprises at least one detector configured fordetecting the different interference light patterns. In one embodiment,the detector(s) comprises an array of detectors configured forsimultaneously detecting spatial components of each differentinterference light pattern. In this case, each detector respectivelystores a plurality of values in a plurality of bins representative ofthe respective spatial components of the interference light patterns.Each of the interference light patterns may comprise a speckle lightpattern, in which case, the spatial components may comprise specklegrains of the speckle light pattern. The array of detectors may beconfigured for simultaneously detecting spatial components of eachdifferent interference light pattern, and storing the plurality ofvalues for all of the interference patterns in the plurality of binswithin 10 milliseconds, and preferably within 1 microsecond to 1millisecond. The UOT system may further comprise a lock-in camera thatincludes the array of detectors and corresponding bins.

The UOT system further comprises a processor configured for determininga physiologically-dependent optical parameter (e.g., the level ofdeoxygenated and/or oxygenated hemoglobin concentration or relativeabundance or the level of water concentration or relative waterconcentration of brain matter of brain matter) based on the plurality ofvalues stored in the bins. In one embodiment, the processor isconfigured for determining neural activity within the target voxel basedon the determined physiologically-dependent optical parameter. In oneembodiment, the processor is configured for reconstructing the amplitudeof the signal light using the plurality of values stored in each of thebins, and determining the physiologically-dependent optical parameter ofthe target voxel based on the reconstructed amplitude of the signallight. Each value may be respectively stored in each of the bins as anintensity of the spatial component of the respective interference lightpattern, in which case, the processor may be configured for using theplurality of values stored in each of the bins to extract a product ofthe amplitude of the signal light and a known amplitude of the referencelight, and determining the amplitude of the signal light from theextracted product.

In accordance with a second aspect of the present inventions, a methodof performing pulsed UOT comprises delivering ultrasound into a targetvoxel (e.g., one comprising brain matter) within an anatomicalstructure. The target voxel may be relatively small, e.g., less than onemm³.

The method further comprises delivering sample light into the anatomicalstructure, whereby a portion of the sample light passing through thetarget voxel is scattered by the anatomical structure as signal light,and another portion of the sample light not passing through the targetvoxel is scattered by the anatomical structure as background light thatcombines with the signal light to create a sample light pattern.

The method further comprises pulsing the ultrasound and the sample lightin synchrony, such that only the signal light is frequency shifted bythe ultrasound. Multiple pulses of the sample light are delivered intothe anatomical structure for each pulse of the ultrasound delivered intothe target voxel. In one method, the pulses of the sample light areidentical. For example, the identical pulses of the sample light may begenerated from a single optical pulse.

The method further comprises combining reference light with the samplelight pattern to generate an interference light pattern. The method mayfurther comprise generating source light, and splitting the source lightinto the sample light and the reference light. The reference light maybe combined with the signal light in a homodyne manner. For example, themethod may further comprise frequency shifting the sample light by thefrequency of the ultrasound, such that the reference light is combinedwith the signal light in the homodyne manner.

The method further comprises sequentially modulating the interferencelight pattern to generate a plurality of different interference lightpatterns. In one method, the interference light pattern may besequentially modulated by phase modulating the interference lightpattern. For example, the interference light pattern may be phasemodulated by setting different phase differences (e.g., 0, π/2, π, and3π/2) between sequential pulses of the sample light and the referencelight.

The method further comprises detecting the different interference lightpatterns. In one method, spatial components of each differentinterference light pattern are simultaneously detected. In this case,the method may further comprise storing a plurality of values in aplurality of bins representative of the respective spatial components ofthe interference light patterns. Each of the interference light patternsmay comprise a speckle light pattern, in which case, the spatialcomponents may comprise speckle grains of the speckle light pattern. Thespatial components of each different interference light pattern may besimultaneously detected, and the plurality of values for all of theinterference patterns may be stored in the plurality of bins within 10milliseconds, and preferably within 1 microsecond to 1 millisecond.

The method further comprises determining a physiologically-dependentoptical parameter (e.g., the level of deoxygenated and/or oxygenatedhemoglobin concentration or relative abundance or the level of waterconcentration or relative water concentration of brain matter of brainmatter) of the target voxel based on the plurality of values stored inthe bins. One method further comprises determining neural activitywithin the target voxel based on the determinedphysiologically-dependent optical parameter.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a block diagram of an ultrasound modulating optical tomography(UOT) system constructed in accordance with one embodiment of thepresent inventions;

FIG. 2 a block diagram of one embodiment of an acoustic assembly used inthe UOT system of FIG. 1;

FIG. 3a is a block diagram of one embodiment of an interferometer usedin the UOT system of FIG. 1;

FIG. 3b is a block diagram of another embodiment of an interferometerused in the UOT system of FIG. 1;

FIG. 3c is a block diagram of still another embodiment of aninterferometer used in the UOT system of FIG. 1;

FIG. 3d is a block diagram of yet another embodiment of aninterferometer used in the UOT system of FIG. 1;

FIG. 4 is a schematic diagram of one embodiment of a detector array usedin the UOT system of FIG. 1;

FIG. 5 is a timing diagram of one pulsing sequence used by the UOTsystem to detect a physiologically-dependent optical parameter in atarget voxel within an anatomical structure;

FIG. 6 is a schematic diagram of the UOT system of FIG. 1, particularlyshowing the generation of interference light patterns, the detection ofspatial components in the, and binning of spatial component values;

FIG. 7 is a timing diagram of another pulsing sequence used by the UOTsystem to detect a physiologically-dependent optical parameter in atarget voxel within an anatomical structure;

FIG. 8 is a plan diagram of one embodiment of an optical assembly usedto split a single optical pulse into a train of identical optical pulsesfor use in the UOT system of FIG. 1;

FIG. 9a is a schematic diagram of a modified UOT system of FIG. 1,particularly showing a detection of a wavefront of signal light;

FIG. 9b is a schematic diagram of a modified UOT system of FIG. 1,particularly showing playback of a phase conjugate of the wavefront ofsignal light;

FIG. 10a is a block diagram of one embodiment of a phase conjugationarray that can be incorporated into the UOT system of FIG. 1,particularly showing detection of the signal light;

FIG. 10b is a block diagram of the phase conjugation array of FIG. 10 a,particularly showing playback of a phase conjugation light field;

FIG. 11 is a plot of the absorption of light in water over thewavelength of light;

FIG. 12 is a plan view of wearable and unwearable units in which the UOTsystem of FIG. 1 may be embodied;

FIG. 13 is a flow diagram of one method used by the UOT system of FIG. 1to non-invasively measure a physiologically-dependent optical parameterusing the pulse sequence of FIG. 5; and

FIG. 14 is a flow diagram of another method used by the UOT system ofFIG. 1 to non-invasively measure a physiologically-dependent opticalparameter using the pulse sequence of FIG. 7.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The ultrasound modulated optical tomography (UOT) systems describedherein utilize the combination of a pulsed ultrasound sequence that tagslight propagating through an anatomical structure, and a selectivelock-in camera that detects the tagged light (e.g., via parallel speckledetection (PSD)), as opposed to a conventional camera, to provide ahighly efficient and scalable scheme that enables detection of highlylocalized and high spatial resolution UOT signals (e.g., blood-oxygenlevel dependent signals) at great depth inside a biological specimen,e.g., noninvasively through the entire thickness of the human skull andinto the underlying cerebral cortical brain matter. The UOT systems mayutilize a specific homodyne interference scheme that enables shot noiselimited detection of the signal light. Such UOT signals may be used for,e.g., brain-computer interfacing, medical diagnostics, or medicaltherapeutics. Although the UOT systems are described herein as beingused to image brain tissue for exemplary purposes, such UOT system canbe used to image other anatomical parts of the body.

Referring to FIG. 1, an ultrasound modulated optical tomography (UOT)system 10 constructed in accordance with one embodiment of the presentinventions will be described. The UOT system 10 is designed tonon-invasively measure a physiologically-dependent optical parameter ofa target voxel 14 in an anatomical structure 16. In the illustratedembodiment, the anatomical structure 16 is the intact head of a patient18 (shown in FIG. 12), including the scalp, skull, and brain, with thetarget voxel 14 being a portion of the brain. In a practicalimplementation, the UOT system 10 will acquire data from multiple targetvoxels 14 (“data voxels”) spatially separated from each other within avolume of interest (not shown). A “target voxel” may be defined as asmall contiguous sub-volume of space (e.g., a cube) within theanatomical structure 16. For purposes of brevity, the UOT system 10 willbe described as acquiring one data voxel (i.e., data representative of aphysiologically-dependent optical parameter of the target voxel 14),although it should be understood that the UOT system 10 may be capableof acquiring more than one data voxel from the volume of interest of theanatomical structure 16.

In the illustrated embodiment, the physiologically-dependent opticalparameter may be, e.g., a level of deoxygenated and/or oxygenatedhemoglobin concentration or relative abundance, although in otherembodiments, the physiologically-dependent optical parameter can be anyparameter that varies in accordance with a change in an optical propertyof the target voxel 14 (e.g., light absorption). Thephysiologically-dependent optical parameters may alternatively comprisean analyte concentration in the blood, analyte/metabolite in tissue,concentration of a substance (e.g., blood, hemoglobin) or a structurewithin tissue, the presence and concentration of lamellar bodies inamniotic fluid for determining the level of lung maturity of a fetus,the presence and/or concentration of meconium in the amniotic fluid,optical properties of other extravascular fluids, such as pleural,pericardial, peritoneal, and synovial fluids. Thephysiologically-dependent optical parameter may be used internallywithin the UOT system 10 or may be transmitted to external devices foruse therein, e.g., medical devices, entertainment devices,neuromodulation stimulation devices, alarm systems, video games, etc.

The UOT system 10 generally includes an acoustic assembly 20, aninterferometer 22, a controller 24, a lock-in camera 28, and a processor30.

The acoustic assembly 20 is configured for delivering ultrasound 32 intothe target voxel 14. Preferably, the acoustic assembly 20 focuses theultrasound 32 on this target voxel 14 in order to maximize the imagingresolution of the UOT system 10; that is, the more focused theultrasound 32 is, the smaller the target voxel 14 may be defined,thereby increasing the resolution of the UOT system 10.

Preferably, the frequency f_(us) of the ultrasound 32 is selected (e.g.,in the range of 100 KHz-10 MHz), such that the ultrasound 32 can passefficiently through the skull and brain matter without significantattenuation that would otherwise cause insufficient ultrasound pressureat the target voxel 14, so that detectable UOT modulation of the lightis created, as described in further detail below. It should beappreciated that the wavelength of such ultrasound in brain matter,given that the speed of sound in brain matter is similar to that ofwater (1500 meter/second), is on the order of fractions of a millimeterto a few millimeters. Thus, the acoustic assembly 20 may obtainultrasound focal confinement at the target voxel 14 laterally on theorder of the wavelength of the ultrasound 32 (e.g., less than 1 mm), andaxially on the order of the wavelength of the ultrasound 32 when theacoustic assembly 20 is operated to pulse the ultrasound 32 at shortdurations (e.g., a single cycle), as will be described in further detailbelow.

Referring further to FIG. 2, one embodiment of the acoustic assembly 20includes an ultrasound transducer arrangement 34 and a signal generator36. The ultrasound transducer arrangement 32 may take the form of anydevice that emits ultrasound 32 (in the illustrated embodiment, focusedultrasound) at a defined frequency and duration in response to acontrolled drive signal; for example, signal acoustic element configuredfor emitting ultrasound beams with a fixed focus; or a piezoelectricphased array capable of emitting ultrasound beams with variabledirection, focus, duration, and phase, or may be an array of pressuregenerating units (e.g., silicon, piezoelectric, polymer or other units),an ultrasound imaging probe, or even an array of laser generatedultrasound (LGU) elements.

The signal generator 36 is configured for generating alternating current(AC) signals for driving the ultrasound transducer arrangement 34 at adefined ultrasound frequency, duration, and intensity. The AC drivesignal may be electrical or optical, depending on the nature of theultrasound transducer arrangement. The signal generator 36 includescontrol inputs (not shown) for receiving control signals from thecontroller 24 that cause the ultrasound transducer arrangement 34 toemit the ultrasound 32 at a selected time, duration, and intensity.Thus, as will be described in further detail below, the controller 24may selectively pulse the ultrasound 32.

In one particular embodiment, the transducer arrangement 34 is ahead-mounted steerable ultrasonic array coupled to the skin of thepatient via hydrogel or other means of mechanical coupling in order toeffectively launch the ultrasound 32 towards the precisely definedtarget voxel 14 within the anatomical structure 16, and in this case,the three-dimensional volume of the brain, while compensating theultrasound wavefront using well-known phased array techniques to achieveefficient and selective ultrasound delivery to the target voxel 14.

Referring to FIGS. 1 and 3, the interferometer 22 is configured fordelivering sample light 40 into the anatomical structure 16, where itscatters diffusively, e.g., through the human skull, into the brain, andback out again. Thus, a portion 40 a of the sample light 40 will passthrough the target voxel 14 and will be scattered by the anatomicalstructure 16 as signal light 44, and another portion 40 b of the samplelight 40 will not pass through the target voxel 14 and will be scatteredby the anatomical structure 16 as background light 46. The signal light44 and background light 44 combine together to create a sample lightpattern 47 that exits the anatomical structure 16. The interferometer 22is further configured for combining reference light 42 with the samplelight pattern 47 to generate an interference light pattern 48 (e.g.,speckle light pattern, which can be defined as an intensity patternproduced by the mutual interference of a set of scattered wavefronts;that is, a speckle light pattern results from the interference of manywaves, but having different phases and amplitudes, which add together togive a result wave whose amplitude, and therefore intensity and phase,varies randomly). In the illustrated embodiment, the interferometer 22is configured for splitting source light 38 into the sample light 40 andreference light 42, as will be described in further detail below.

The reference light 42 may be combined with the signal light 44 in thesample light pattern 47 in a homodyne manner, e.g., by initiallyfrequency shifting the sample light 40 by the frequency f_(us) of theultrasound 32 delivered into the target voxel 14 by the acousticassembly 20. That is, if unmodified, the sample light portion 40 apassing through the target voxel 14 will be frequency shifted (i.e.,tagged) by the ultrasound 32 that also passes through the target voxel14, such that the signal light 44 will have frequencies f−f_(us).Presumably, the sample light portion 40 b not passing through the targetvoxel 14 will not be frequency shifted (i.e., untagged) by theultrasound 32, such that the background light 46 will have a frequencyf, i.e., the frequency of the sample light 40. It is also that not allof the sample light portion 40 a passing through the target voxel 14will be tagged by the ultrasound 32 (i.e., there exists a taggingefficiency (i.e., the number of tagged photons relative to a number ofuntagged photons scattered by the target voxel 14)), and therefore, someof the sample light portion 40 a passing through the target voxel 14will be scattered by the anatomical structure 16 as background light 46.

However, assuming that the reference light 42 and the sample light 40output by the interferometer 22 have the same frequency f, in order tocombine the ultrasound tagged signal light 44 in the sample lightpattern 47 and the reference light 42 in a homodyne manner, whichrequires the reference light 42 and signal light 44 to have the samefrequency, the frequency f of the sample light 40 or the reference light42 must initially be shifted relative to each other by the ultrasoundfrequency f_(us), such that, upon combining by the interferometer 22,the frequency of the ultrasound tagged signal light 44 will be shiftedto the same frequency as the reference light 42, and the frequency ofthe untagged background light 46 will differ from the frequency of thereference light 42 by the ultrasound frequency f_(us). Thus, either thesample light 40 or the reference light 42 will be pre-conditioned, suchthat the ultrasound tagged signal light 44 will interfere with thereference light 42 in a homodyne manner, resulting in a DC interferencecomponent between the reference light 42 and signal light 44 that can bedetected by the lock-in camera 28 as the signal component during eachpulse, as will be described in further detail below. In contrast, thefrequency shifting of the sample light 40 before it enters theanatomical structure 16, or the frequency shifting of the referencelight 42, will prevent the untagged background light 46 from interferingwith the reference light 42 in a homodyne manner.

In the embodiment illustrated in FIG. 1, the interferometer 22 downfrequency shifts the sample light 40 by the ultrasound frequency f_(us)to f−f_(us), such that the ultrasound tagged signal light 44 has thefrequency f_(us), the untagged background light 46 has the frequencyf−f_(us), and the reference light 42 has a frequency f_(us), therebyenabling combination of the reference light 42 and signal light 44 in ahomodyne manner, as further described below with respect to FIG. 3 a.However, it is noted that because the ultrasound 32 will tag the signallight 44 with the ultrasound frequencies +f_(us) and −f_(us), as well asother positive and negative multiples of the ultrasound frequencyf_(us), other frequency shifting implementations are possible to effecthomodyne combination of the reference light 42 and signal light 44. Forexample, as described in further detail below, the interferometer 22 mayup frequency shift the sample light 40 by the ultrasound frequencyf_(us) to f+f_(us), such that the ultrasound tagged signal light 44 hasthe frequency f_(us), the untagged background light 46 has the frequencyf+f_(us), and the reference light 42 has a frequency f_(us) (see FIG. 3b); may up frequency shift the reference light 42 by the ultrasoundfrequency f_(us) to f+f_(us), such that the ultrasound tagged signallight 44 has the frequency f+f_(us), the untagged background light 46has the frequency f, and the reference light 42 has a frequency f+f_(us)(see FIG. 3c ); may down frequency shift the reference light 42 by theultrasound frequency f_(us) to f−f_(us), such that the ultrasound taggedsignal light 44 has the frequency f−f_(us), the untagged backgroundlight 46 has the frequency f, and the reference light 42 has a frequencyf−f_(us) (see FIG. 3d ); or perform any other frequency shift of thesample light 40 or reference light 42 that results in the homodynecombination of the reference light 42 and the signal light 44.

The interferometer 22 is further configured for modulating (and in theillustrated embodiment, phase modulating) the interference light patternto generate a plurality of different interference light patterns, whichas will be described in further detail below, enables the amplitude ofthe signal light 44 to be distinguished from the background light 46.

Referring further to FIG. 3 a, one embodiment of the interferometer 22includes a light source 50, a beam splitter 52, an optical phase shifter54, an optical frequency shifter 56, a light combiner 58, a path lengthadjustment mechanism 60, and a set of mirrors 62 a, 62 b (generally,62).

The light source 50 is configured for generating coherent light as thesource light 38, preferably at a single wavelength (e.g., in the rangeof 605 nm to 1300 nm), and may take the form of, e.g., a laser diode. Inalternative embodiments, multiple light source(s) (not shown) may beused to generate the source light 38 at multiple distinct wavelengths,e.g., one generating source light 38 within the range of 605 nm to 800nm, and another generating source light 38 within the range of 800 nm to1300 nm. The coherence length of the source light 38 is preferably atleast one meter in order to generate the best speckle contrast in thespeckle light pattern 48. The light source 50 may receive power from adrive circuit (not shown), which may include control inputs forreceiving control signals from the controller 24 that cause the lightsource 50 to emit the source light 38 at a selected time, duration, andintensity. Thus, as will be described in further detail below, thecontroller 24 may selectively pulse the source light 38, and thus thesample light 40 and reference light 42.

As specifically illustrated in FIG. 3 a, the beam splitter 52 isconfigured for splitting the source light 38 into the sample light 40that propagates along a sample arm of the interferometer 22 andreference light 42 that propagates along a reference arm of theinterferometer 22. In the illustrated embodiment, the beam splitter 52(e.g., a partially transparent mirror) splits the source light 38 viaamplitude division by reflecting a portion of the source light 38 as thesample light 40, and transmitting the remaining portion of the sourcelight 38 as the reference light 42, although the beam splitter 52 mayalternatively reflect a portion of the source light 38 as the referencelight 42, and transmit the remaining portion of the source light 38 asthe sample light 40. In alternative embodiments, the beam splitter 52may split the source light 38 via wavefront division by splitting aportion of the wavefront into the sample light 40 and splitting theremaining portion of the wavefront into the reference light 42. Ineither case, the beam splitter 52 may not necessarily split the sourcelight 38 equally into the sample light 40 and reference light 42, and itmay actually be more beneficial for the beam splitter 52 to split thesource light 38 unevenly, such that the amplitude of the sample light 40is less than the amplitude of the reference light 42 (e.g., 10/90 powerratio) in order to comply with tissue safety standards. That is, theamplitude of the sample light 40 will preferably be relatively low toavoid damaging the tissue, whereas the amplitude of the reference light42, which will be used to boost the signal light 44 in the interferencelight pattern 46, will be relatively high.

The optical phase shifter 54 is configured for setting the phasedifference between the sample light 40 and reference light 42. Theoptical phase shifter 54 may include control inputs (not shown) forreceiving control signals from the controller 24 that cause the opticalphase shifter 54 to set the phase of the reference light 42 relative tothe sample light 40. Thus, as will be described in further detail below,the controller 24 may selectively set the phase between the sample light40 and the reference light 42.

The optical frequency shifter 56 is configured for down frequencyshifting the sample light 40 by the ultrasound frequency f_(us) tof−f_(us), such that the frequency of the ultrasound tagged signal light44 will be f, while the frequency of the background light 46 will bef−f_(us), thereby enabling the homodyne combination of the referencelight 42 at frequency f and the ultrasound tagged signal light 44 atfrequency f, as described above with respect to FIG. 1. In onealternative embodiment illustrated in FIG. 3 b, the optical frequencyshifter 56 is configured for up frequency shifting the sample light 40by the ultrasound frequency f_(us) to f+f_(us), such that the frequencyof the ultrasound tagged signal light 44 will be f, while the frequencyof the background light 46 will be f +f_(us), thereby enabling thehomodyne combination of the reference light 42 at frequency f and theultrasound tagged signal light 44 at frequency f. In one alternativeembodiment illustrated in FIG. 3 c, the optical frequency shifter 56 isconfigured for up frequency shifting the reference light 42 by theultrasound frequency f_(us) to f+f_(us), such that the frequency of theultrasound tagged signal light 44 will be f+f_(us), while the frequencyof the background light 46 will be f, thereby enabling the homodynecombination of the reference light 42 at frequency f+f_(us) and theultrasound tagged signal light 44 at frequency f+f_(us). In stillanother alternative embodiment illustrated in FIG. 3 d, the opticalfrequency shifter 56 is configured for down frequency shifting thereference light 42 by the ultrasound frequency f_(us) to f−f_(us), suchthat the frequency of the ultrasound tagged signal light 44 will bef−f_(us), while the frequency of the background light 46 will be f,thereby enabling the homodyne combination of the reference light 42 atfrequency f−f_(us) and the ultrasound tagged signal light 44 atfrequency f−f_(us).

In any event, the frequency shifter 54 may include a local oscillator(not shown) that outputs a signal having a fixed or variable frequency.The local oscillator may be variable, in which case, it may have acontrol input for receiving control signals from the controller 24 thatcause the local oscillator to output a signal at a defined frequency.Alternatively, the local oscillator may be fixed, in which case, it willoutput a signal having a fixed frequency. In either case, the frequencyof the signal output by the local oscillator will be equal to thefrequency f_(us) of the ultrasound 32 emitted by the acoustic assembly20.

The light combiner 58 is configured for combining the reference light 42with the sample light pattern 47 via superposition to generate theinterference light pattern 48. The light combiner 58 can take the formof, e.g., a combiner/splitter mirror.

The path length adjustment mechanism 60 is configured for adjusting theoptical path length of the reference light 42 (i.e., the reference arm)to nominally match the expected optical path length of the combinedsample light 40 and signal light 44 (i.e., the sample arm), such thatthe signal light 44 and the reference light 42 reach the light combiner58 at the same time. The path length adjustment mechanism 60 may includea beam splitter/combiner 64 and an adjustable mirror 66 that can bedisplaced relative to the beam splitter/combiner 64. The beam/splittercombiner 64 is configured for redirecting the reference light 42 at aninety-degree angle towards the mirror 66, and redirecting the referencelight 42 reflected back from the mirror 66 at a ninety-degree angletowards the light combiner 58. Thus, adjusting the distance between themirror 66 and the beam splitter/combiner 64 will adjust the optical pathlength of the reference arm to match the optical path length of thesample arm.

The mirror assembly 62 is configured for confining the optical lightpaths in the interferometer 22 into a small form factor, and in theillustrated embodiment, includes a first tilted, completely reflective,mirror 62 a configured for redirecting the sample light 40 at aninety-degree angle towards the biological specimen 16, and a secondtilted, completely reflective, mirror 62 b configured for redirectingthe signal light 44 (and coincidentally a portion of the backgroundlight 46) towards the light combiner 58.

Referring back to FIG. 1, the controller 24, which may, e.g., take theform of a central processing unit (CPU), is configured for implementingpulsed wave (PW) UOT by operating the acoustic assembly 20 to pulse theultrasound 32 (in the illustrated embodiment, by sending on/off controlsignals to the signal generator 36), and operating the interferometer 22to pulse the sample light 40 (in the illustrated embodiment, by sendingon/off control signals to the drive circuit coupled to the light source50) in synchrony with the (comparatively very slow) flight of theultrasound 32, such that only the signal light 44 is frequency shifted(i.e., tagged) by the ultrasound 32. That is, a pulse of the samplelight 40 will be delivered into the anatomical structure 16, such thatit will pass through the target voxel 14 only as the ultrasound 32passes through the target voxel 14. In this manner, no portion of thebackground light 46 will be tagged by the ultrasound 32. As a result,pulsed wave (PW) UOT improves the spatial resolution in the axialdirection (or depth) compared to continuous wave (CW) UOT. Thus, PW UOTachieves axial confinement and three-dimensional (3D) spatialresolution, rather than merely two-dimensional (2D) spatial resolutionas in the case with CW UOT.

The controller 24 is further configured for operating the interferometer22 to sequentially modulate the interference light pattern 48 (in theillustrated embodiment, by sending on/off control signals to the opticalphase shifter 54) to generate a plurality of different interferencelight patterns. As will be described in further detail below, theinterferometer 22 will set different phases (and in the illustratedembodiment, four different phases equal to 0, π/2, π, and 3π/2) betweensequential pulses of the sample light 40 and the reference light 42 tofacilitate quadrature detection of the signal light 44.

As will be also described in further detail below, the controller 24 isfurther configured for synchronously operating the lock-in camera 28,such that the bin shifting of data detected by the lock-in camera 28 isperformed in synchrony with the phase changes in the interferometer 22.

Referring further to FIG. 4, the lock-in camera 28 includes an array ofdetectors 68 (or “pixels”) configured for simultaneously detectingspatial components of each of the different interference light patterns48. In the case where the interference light pattern 48 is a specklelight pattern, the spatial components are speckle grains (approximatelythe size of a wavelength of the light) of the speckle light pattern. Ingeneral, lock-in cameras include a class of digital cameras in whichmultiple measurements of a light field are rapidly made at each pixel ina temporally precise fashion synchronized with an external trigger oroscillation and stored in multiple “bins” within each pixel, in contrastwith conventional cameras, which store only one value per pixel thatmerely aggregate the incoming photo-electrons over the camera frameintegration time. Lock-in cameras may also perform on-chip computationson the binned values. Thus, the key feature of lock-in cameras is theirability to rapidly capture and store multiple sequential samples of thelight field, with sample-to-sample latencies shorter than readout timesof conventional cameras. This feature enables them, for example, tosample a modulated light field at the same frequency as the modulation,such that subtraction across successive samples, or other operations,such as quadrature detection (discussed below) will extract thecomponent of the light that is modulated at the modulation frequency,while subtracting off the unmodulated (“DC”) background. Similarly,lock-in cameras can be used to make a series of such measurements orcomparisons, locked to an external trigger signal (generated by thecontroller 24), rapidly in order to extract such modulated componentsfrom a rapidly changing light field arising from a dynamic, disorderedbiological specimen.

Thus, each detector 68 of the lock-in camera 28 respectively stores aplurality of values in a plurality of bins 70 a-70 d representative ofthe spatial component of the four interference light patterns 48, and inthis case, four bins 70 a-d (in general, 70) for storing four valuesfrom the respective four interference light patterns 48. The spatialcomponent values stored in the bins 70 of a respective detector 68 maybe, e.g., the intensity values of the respective spatial component ofinterference light patterns 48. For example, for any particular detector68 (or pixel) corresponding to a particular spatial component (orspeckle grain), four power values P_(a)-P_(d) for the four interferencepatterns 48 will be respectively stored in the four bins 70 a-70 d. Aswill be described in further detail below, the spatial component powervalues P_(a)-P_(d) detected by each detector 68 of the camera 28 for thefour interference patterns 48 can be used to reconstruct the amplitudeof the signal light 44, and thus, can be said to be representative ofthe physiologically-dependent optical parameters (e.g., opticalabsorption) of the target voxel 14. The lock-in camera 28 includescontrol inputs (not shown) for receiving control signals from thecontroller 24, such that the detection and binning of the data can becoordinated with the pulsing of the ultrasound 32 and sample light 40described in further detail below.

Although only a single lock-in camera 28 is illustrated, it should beappreciated that multiple lock-in cameras 28 (e.g., in an array) or alock-in camera in the form of multiple camera sensor chips on a commoncircuit board, can be used to increase the number of detectors 68 (i.e.,pixels). Although not illustrated, the system 10 may includemagnification optics and/or apertures to magnify the individual specklegrains, which may have a size on the order of the wavelength of thenear-infrared or visible light used to acquire the data voxel, and henceon the order of hundreds of nanometers in size, to approximately thesizes of the detectors 68 of the lock-in camera 28. Thus, in theillustrated embodiment, the pixel sizes and pitches of the lock-incamera 28 are matched to the speckle grain sizes and pitches of theinterference light pattern 48 via the appropriate magnification,although other embodiments are possible.

Referring to FIGS. 5 and 6, one pulsing sequence that can be used in aPW UOT technique performed by the system 10 for generating fourinterference light patterns 48 and detecting and storing the spatialcomponent power values for the interference light patterns 48 will bedescribed.

During one acquisition of a single data voxel (i.e., acquisition of datacharacterizing the target voxel 14), an ultrasound pulse trainconsisting of four separate, but identical, ultrasound pulsesU_(a)-U_(d) are delivered into the target voxel 14. In this embodiment,the duration τ of each ultrasound pulse U is equal to only one fullcycle of the ultrasound 32 to maximize the data acquisition speed, andthus, is equal to 1/f_(us), although in alternative embodiments, theduration τ may be several ultrasound cycles long (e.g., on the order of1 microsecond or less than one microsecond). It should be noted that itis desirable to minimize the duration τ of the ultrasound pulse U inorder to minimize ultrasound focal confinement at the target voxel 14.

The duty cycle of the ultrasound pulses U_(a)-U_(d) (i.e., the time thatelapses between the beginning of one pulse U to the beginning of thenext pulse U) is τ_(duty). The duty cycle τ_(duty) may be selected toallow each ultrasound pulse U to exit the anatomical structure 16 beforethe next measurement is taken, such that the ultrasound tagged signallight 44 is only present at high pressures at the three-dimensionallocation of the target voxel 14. The frame rate of the lock-in camera 28should be selected to match the duty cycle τ_(duty) of the ultrasoundpulse U, such that there exists one ultrasound pulse U per frame.

A light pulse train consisting of four sample light pulses L_(a)-L_(d)is also delivered into the anatomical structure 16 in synchrony with thedelivery of the four ultrasound pulses U_(a)-U_(d), such that, as eachultrasound pulse U passes through the target voxel 14, the sample lightpulse L likewise passes through the target voxel 14.

In this manner, only the signal light 44 (and none of the backgroundlight 46) is tagged with the ultrasound, as discussed above. In thisparticular embodiment, only one sample light pulse L is delivered foreach ultrasound pulse U. Thus, there is a one-to-one correspondencebetween the sample light pulses L_(a)-L_(d) and the ultrasound pulsesU_(a)-U_(d). Although each of the sample light pulses L is illustratedin FIG. 5 as having the same width as the width of the respectiveultrasound pulses U for purposes of illustration, each sample lightpulses L is, in practicality, at least slightly smaller than therespective ultrasound pulse U due to the latency period required for therespective ultrasound pulse to reach the target voxel 14. Alternatively,the duration of the sample light pulse L, the duration of the samplelight pulse L can be much less than the duration of the ultrasound pulseU. In any event, the duration of the sample light pulse L preferablyshould be approximately matched to be an integer multiple of thefrequency of the ultrasound frequency f_(us), e.g., if the ultrasoundfrequency f_(us) of 2 MHz, the duration of the sample light pulse Lshould be multiples of 0.5 microseconds. Although FIG. 5 illustratesonly one ultrasound cycle per ultrasound pulse U, if the duration of thesample light pulse L is multiple integers of the frequency of theultrasound frequency f_(us), the number ultrasound cycles per ultrasoundpulse U is preferably equal to the same multiple integer. This ensuresthat a full, balanced cycle of tagged light is generated. The energy ofeach sample light pulse L should be sufficiently high, e.g., on theorder of 1 microsecond in duration (but can be as low as 10 nanosecondsin duration) and tens of micro-Joules per square centimeter.

For each of the four separate ultrasound pulses U_(a)-U_(d) occurringduring the acquisition of a single data voxel, the phase differencebetween the reference light 42 and the sample light 40 is set to adifferent setting, and in this case, to one of 0, π/2, π, and 3π/2. Inthe illustrated embodiment, the phase between the reference light 42 andthe sample light 40 is sequentially set to 0, π/2, π, and 3π/2, althoughthese phase settings can be performed in any order, as long as all fourphase settings 0, π/2, π, and 3π/2 are used during the acquisition of asingle data voxel.

The respective pulses of the sample light pattern 47 and reference light42 are then combined into the interference light patterns 48, eachhaving four corresponding interference pulses I_(a)-I_(d) that can bedetected by the lock-in camera 28. That is, for each interference pulseI, a detector 68 detects a spatial component of the respectiveinterference pulse I (e.g., a speckle grain in the case where theinterference light pattern 48 includes a speckle pattern) and stores thespatial component value (e.g., power) within a respective one of thebins 70.

That is, at phase φ=0, a given pixel n will detect and store the valueof the respective spatial component of the interference pulse I_(a) intobin 1 of the pixel n; at phase φ=π/2, the pixel n will detect and storethe value of the respective spatial component of the interference pulseI_(b) into bin 2 of the pixel n; at phase φ=π, the pixel n will detectand store the value of the respective spatial component of theinterference pulse I_(c) into bin 3 of the pixel n; and at phase φ=3π/2,the pixel n will detect and store the value of the respective spatialcomponent of the interference pulse I_(d) into bin 4 of the pixel n.

Similarly, at phase φ=0, the next pixel n+1 will detect and store thevalue of the respective spatial component of the interference pulseI_(a) into bin 1 of the pixel n+1; at phase φ=π/2, the pixel n+1 willdetect and store the value of the respective spatial component of theinterference pulse I_(b) into bin 2 of the pixel n+1; at phase φ=π, thepixel n+1 will detect and store the value of the respective spatialcomponent of the interference pulse into bin 3 of the pixel n+1; and atphase φ=3π/2, the pixel n+1 will detect and store the value of therespective spatial component of the interference pulse I_(d) into bin 4of the pixel n+1.

Similarly, at phase φ=0, the next pixel n+2 will detect and store thevalue of the respective spatial component of the interference pulseI_(a) into bin 1 of the pixel n+2; at phase φ=π/2, the pixel n+2 willdetect and store the value of the respective spatial component of theinterference pulse I_(b) into bin 2 of the pixel n+2; at phase φ=π, thepixel n+2 will detect and store the value of the respective spatialcomponent of the interference pulse into bin 3 of the pixel n+2; and atphase φ=3π/2, the pixel n+2 will detect and store the value of therespective spatial component of the interference pulse I_(d) into bin 4of the pixel n+2.

Thus, for each of an n number of pixels, four values will berespectively stored in the four bins 1-4. Significantly, in the casewhere the interference light pattern 48 includes a speckle lightpattern, it is important that all four sample light pulses P bedelivered by the interferometer 22 to the target voxel 14 and that allfour interference pulses I be detected and recorded by the camera 28within the characteristic speckle decorrelation time of the target voxel14, which scales super-linearly with the depth into the anatomicalstructure 16 at which the target voxel 14 is located. For imaging deepinside a living biological tissue, such as through the human skull andinto the human cerebral cortex, the speckle decorrelation time isexpected to be on the order of microseconds to tens of microseconds. Forimaging directly into living brain matter in the absence of skull,speckle decorrelation times have been measured to be on the order of tenmilliseconds for 1-millimeter penetration or 1-millisecond for3-millimeter penetration. Notably, the speckle decorrelation timeimpacts the depth scaling of lock-in camera based UOT in dynamicscattering media, such as biological tissue, namely the constraint thatmultiple phase-shifted measurements must be made within the speckledecorrelation time (see, e.g., Qureshi M M, Brake J., Jeon H J, Ruan H,Liu Y, Safi A M, Eom T J, Yang C., Chung E, “In Vivo Study of OpticalSpeckle Decorrelation Time Across Depths in the Mouse Brain,” BiomedicalOptics Express, Vol. 8, No. 11, pp. 4855-4864 (Nov. 1, 2017). Thus, itis important that the time window in which the set of quadraturemeasurements is short enough that the target voxel 14 does not have thetime to de-correlate significantly. Otherwise, the signal-to-noise ratiois diminished.

Referring to FIG. 7, another particularly advantageous pulsing sequencethat can be used in a PW UOT technique performed by the system 10 forgenerating four interference light patterns 48 and detecting and storingthe spatial component power values for the interference light patterns48 will be described. The pulsing sequence of FIG. 7 is identical to thepulsing sequence of FIG. 5, with the exception that multiple samplelight pulses L, and in this embodiment, all four sample light pulses L,are delivered to the target voxel 14 for each ultrasound pulse Udelivered to the target voxel 14, thereby accelerating the delivery ofthe sample light pulses P by the interferometer 22 to the target voxel14, and the resultant generation, detection, and recording of all fourinterference pulses I by the camera 28. That is, because the four samplelight pulses L are delivered for each ultrasound pulse U, the speed ofthe data voxel acquisition is increased by a factor of four.

In particular, during the acquisition of four consecutive data voxels(as opposed to only one in the pulsing sequence of FIG. 5), anultrasound pulse train consisting of four separate, but identical,ultrasound pulses U are delivered into the target voxel 14. As with thecase in the pulsing sequence of FIG. 5, the duration τ of thisultrasound pulse U is equal to only one full cycle of the ultrasound 32to maximize the data acquisition speed, and thus, is equal to 1/f_(us).The duration τ of this ultrasound pulse U and the duty cycle τ_(duty) ofthe ultrasound train pulse of FIG. 7 can be identical to the respectiveduration τ and duty cycle τ_(duty) of the ultrasound pulse train in thepulsing sequence of FIG. 7.

A light pulse train consisting of four sets of sample light pulses, witheach set comprising four sample light pulses L_(a)-L_(d), are alsodelivered into the anatomical structure 16 in synchrony with thedelivery of the four ultrasound pulses U, such that as each ultrasoundpulse U passes through the target voxel 14, the corresponding set offour sample light pulses L_(a)-L_(d), likewise pass through the targetvoxel 14. Thus, only the signal light 44 (and none of the backgroundlight 46) is tagged with the ultrasound, as discussed above. Thus, foursample light pulses L_(a)-L_(d) are delivered for each ultrasound pulseU. Thus, there is a four-to-one correspondence between the sample lightpulses L_(a)-L_(d) and ultrasound pulses U.

Thus, in the same manner described above with respect to the pulsingsequence illustrated in FIG. 5, for each ultrasound pulse U occurringduring the acquisition of a single data voxel, the phase differencebetween the reference light 42 and the sample light 40 is set to adifferent setting, and in this case, to one of 0, π/2, π, and 3π/2, togenerate four interference pulses I_(a)-I_(d). The quick detection andstorage scheme of the lock-in camera 28 enables acquisition of an entiredata voxel within one cycle of the ultrasound 32, well within thespeckle decorrelation time of the target voxel 14.

It can be appreciated that the use of the lock-in camera 28 provides fora high-speed and precisely timed detection method that can capturedifferences in a light field far faster than the frame rates ofconventional cameras. In the illustrated embodiment, the lock-in camera28 rapidly measures the four quadratures of the pulse sequencesillustrated in FIGS. 5 and 7. The acquisition sequence can be preciselytimed to external signals for integration with optimal ultrasound andlight pulse sequences. The lock-in camera 28 also enables an efficientdetection scheme compared to conventional pulsed UOT, since the pulsedUOT signals may be quickly detected with a high signal-to-noise ratio,while using the full bit depth of the analog-digital conversionavailable in the signal chain due to rejection of DC background by thelock-in camera 28. The lock-in camera 28 provides for a simplifieddetection scheme that is highly scalable to large numbers of pixels andhigh frame rates, enabling the maximization of signal capture in UOT,thereby improving spatial and temporal resolution.

It should be appreciated that in addition to the ability of thecombination of the pulsed UOT with a lock-in camera to provide highaxial spatial resolution and high sensitivity from the high-speedlock-in detection, such combination also provides the additionaladvantage of efficiently detecting the signal light associated with aspecific time point on the ultrasound phase cycle (e.g., at the peaks ofthe ultrasound phase cycle). As such, the pulsed UOT/lock-in cameracombination can accurately image tissue with a relatively small numberof data measurements, and thus, a relatively short period of time,preferably within the speckle decorrelation time of the target voxel. Incomparison, a continuous wave approach results in averaging light signaldetection over a range of arbitrarily placed points on the ultrasoundphase cycle, leading to a diminished overall detection sensitivity,requiring that, for sufficient sensitivity, data measurements be takenover a period time longer than the speckle decorrelation time of thetarget voxel. Thus, the use of pulsed UOT in combination with thelock-in camera allows deeper imaging into tissue.

The detection processes illustrated in FIGS. 5 and 7 require theultrasound timing and intensity to be consistent. As such, the output ofthe acoustic assembly 20 may be periodically sampled to ensure that thesystem 10 does not suffer from ultrasound performance drift, forinstance, as the transducer arrangement 34 heats. Furthermore, thedetection processes illustrated in FIGS. 5 and 7 require that all of thelight pulses used between phase changes in the quadrature measurementsbe equal in strength, or at least exhibit a known ratio that can benormalized out post-detection. In the case that the pulse-to-pulsevariations in the light intensity emitted by the light source 50 are toolarge, the optical arrangement illustrated in FIG. 8 can be utilized togenerate optical pulses that are identical in intensity andintensity-time profile, but temporally separated from each other.

In particular, this optical arrangement includes a first 1×2 fibersplitter 72 a in which a single optical pulse P (generated by the lightsource 50) is input via an optical fiber 74 and split into two identicaloptical pulses P1, P2. Two optical fibers 74 a, 74 b of differentoptical lengths are connected to the respective outputs of the first 1×2fiber splitter 72 a, such that the two identical optical pulses P1, P2respectively propagate within the two optical fibers 74 a, 74 b. Theoptical arrangement further includes a first 2×1 fiber coupler 76 a intowhich the two identical optical pulses P1, P2 are input and combined,and output to a single optical fiber 74 c. By making the lengths of theoptical fibers 74 a, 74 b different from each other, the single opticalpulse P input into the first 1×2 fiber splitter 72 a is effectivelysplit into two identical optical pulses that propagate through thesingle optical fiber 74 c and are spaced out by a time differencedetermined by the optical path length difference between the two opticalfibers 74 a, 74 b. This conveniently enables the creation of two opticalpulses that track each other identically.

Another fiber coupler and pair of optical fibers can be added to createfour identical optical pulses separated from each other in time. Inparticular, the optical arrangement further includes a second 1×2 fibersplitter 72 b to which the single optical fiber 74 c carrying the twoidentical and temporally spaced optical pulses P1, P2 is coupled. Thus,the two identical optical pulses P1, P2 are input into the second 1×2fiber splitter 72 b and split into four identical optical pulses P1 a,P1 b, P2 a, P2 b (i.e., the optical pulse P1 is split into opticalpulses P1 a, P1 b, and the optical pulse P2 is split into optical pulsesP2 a, P2 b). Two optical fibers 74 d, 74 e of different optical lengthsare connected to the respective outputs of the second 1×2 fiber splitter72 b, such that the two sets of two identical optical pulses P1 a, P1 band P2 a, P2 b respectively propagate within the two optical fibers 72d, 72 e. The optical arrangement further includes a second 2×1 fibercoupler 76 b into which the two sets of identical optical pulses P1 a,P1 b and P2 a, P2 b are input and combined, and output to a singleoptical fiber 74 f. By making the lengths of the optical fibers 74 d, 74e different from each other, the two optical pulses input into thesecond 1×2 fiber splitter 72 b are effectively split into four identicaloptical pulses that propagate through the single optical fiber 74 f andspaced out by a time difference determined by the optical path lengthdifference between the two optical fibers 74 d, 74 e. This convenientlyenables the creation of four optical pulses that track each otheridentically.

Referring back to FIG. 1, once the camera 28 acquires the data voxel bystoring all spatial component values of each of the four interferencepulses I_(a)-I_(d) within the four bins 70 of each of the detectors 68,these data can be sent to the processor 30 (which can, e.g., take theform of a computer, field-programmable gate array or applicationspecific integrated circuit), which is configured for determining aphysiologically-dependent optical parameter (e.g., absorption) of thetarget voxel 14 based on the four values stored in the bins 70 of eachdetector 68. As briefly discussed above, the four spatial componentvalues can be power values P_(a)-P_(d), which can be used by theprocessor 30 to reconstruct the amplitude of the signal light 44, andthus, can be said to be representative of the physiologically-dependentoptical parameters (e.g., optical absorption) of the target voxel 14.

The spatial component power values P_(a)-P_(d) for all four interferencelight patterns I_(a)-I_(d) can be used in accordance with known“quadrature detection” methods to reconstruct the amplitude of thesignal light 44, which is proportional to the number of tagged photonsemerging from the target voxel 14 (i.e., the number of photons in thesignal light 44), and thus, can be used to measure optical absorption inthe target voxel 14 (e.g., for the purpose of measuring spatiallylocalized neural activity-correlated changes in the level ofdeoxygenated and/or oxygenated hemoglobin concentration or relativeabundance in the brain, which appear as localized changes in the opticalabsorption of blood). In the illustrated embodiment, it should beunderstood that because of the diffusive scattering of light over largedistances through the brain and skull, the interference light pattern 48detected by the lock-in camera 28 takes the form of a random specklepattern in which each localized speckle grain has a definite, but randomphase offset in the interference light pattern 48 (i.e., a beat pattern)between the reference light 42 and the signal light 44. This results inthe unknown random phases in the beat patterns measured by each detector68 (or pixel) in the equations set forth below.

In particular, the power detected at a single detector 68 (or pixel) foreach optical pulse at one of the four phases can be expressed as:

-   [1]    Value_(1,k)=P_(background)+P_(signal)+P_(reference)+2(P_(signal)×P_(reference))^(1/2)×cos(φ_(control)−φ_(unknown1, speckle k))+2(P_(signal)×P_(background))^(1/2)×cos(2π×f_(us)−φ_(unknown2, speckle k))+2(P_(reference)×P_(background))^(1/2)×cos(2π×f_(us)−φ_(unknown3, speckle k)),    where P_(background) represents light at frequency f−f_(us) that has    not been tagged with the ultrasound 32; P_(signal) represents light    at frequency f that has been tagged with the ultrasound 32;    P_(reference) represents the reference light at frequency f;    φ_(control) is a control phase shift introduced into the reference    light 42 for each detected interference pattern 48;    φ_(unknown1, speckle k), φ_(unknown2, speckle k), and    φ_(unknown3, speckle k) are random phases at the kth speckle grain    at the time of measurement, which originates via multiple scattering    of coherent light inside the tissue.

The terms P_(background)+P_(signal)+P_(reference) are constant acrossall four optical pulses with different control phase values φ_(control).The terms2(P_(signal)×P_(background))^(1/2)×cos(2π×f_(us)−φ_(unknown2))+2(P_(reference)×P_(background))^(1/2)×cos(2π×f_(us)−φ_(unknown3))oscillate at the frequency f_(us), and are not detected by the lock-incamera 28, and thus, can be ignored. As such, equation [1] can bereduced to:

-   [2]    P_(background)+P_(signal)+P_(reference)+2(P_(signal)×P_(reference))^(1/2)×cos(φ_(control)−φ_(unknown)),    which is analogous to the well-known quadrature formula:-   [3] A+B×Cos(φ_(control)+φ_(unknown)), where φ_(control) can be    respectively set to 0, π/2, π, and 3π/2 to create four equations.    Both the amplitude B and the unknown phase φ_(unknown) can be    extracted by solving the resulting four equations using the standard    trigonometric identities.

Thus, the term magnitude of P_(signal)×P_(reference) can be extracted byshifting the control phase φ_(control) successively on each of foursuccessive pulses φ_(control)=0, π/2, π, and 3π/2. Even thoughφ_(unknown) is an unknown and random phase, specific to each pixel,which results from the laser speckle pattern due to light scattering inthe tissue, by measuring and storing each of these four measurements atdifferent control phase values φ_(control), the value of theinterference term 2(P_(signal)×P_(reference))^(1/2) may be extracted viathe known principal of “quadrature detection.” Because the power of thereference signal P_(reference) is known or independently measurable, theinterference term 2(P_(signal)×P_(reference))^(1/2) serves as ameasurement of the power of the signal light P_(signal). Thus, using aknown scaling relationship, the power of the signal light P_(signal) canbe determined (either in the absolute sense or relative sense) from theextracted term interference term 2(P_(signal)×P_(reference))^(1/2).

Because the speckle phases are random, according to the known principlesof parallel speckle detection in UOT or in wavefront measurement fromstrongly scattering media, it is known that a single-pixel detector willnot scale to high signal to noise ratios. In particular, the aggregatesignal over a large single-pixel detector would scale as the square rootof detector size, but so would shot noise in the background, and hencethe signal to noise ratio performance of a large detector would notincrease with detector size. In contrast, as described in the equationsbelow, with lock-in detection at each detector (or pixel), the aggregatesignal scales linearly with the number of pixels, while the aggregatebackground shot noise scales as the square root, and hence signal tonoise performance increases as the square root of the number of pixels,giving a strong advantage for using large numbers of pixels.

It can be assumed that the amplitude of P_(reference) is much greaterthan the amplitude of P_(background), and the amplitude of P_(signal) isnaturally much less than the amplitude of P_(reference), since theultrasound tagged signal light 44 originates from a very small targetvoxel 14 within the tissue and the tagging efficiency (i.e., the numberof tagged photons relative to a number of untagged photons scattered bythe target voxel 14) within that target voxel 14 is a small fraction.Thus, only interference terms containing P_(reference) are significantin the sum representing the intensity measured by each pixel (i.e.,P_(background)+P_(signal)+P_(reference)+2(P_(signal)×P_(reference))^(1/2)×cos(φ_(control)−φ_(unknown1))).

Therefore, the dominant signal source contributing to detection has thefollowing number of photons impinging on one pixel:

-   [4] dominant signal=(ε/hv)×2(P_(signal)×P_(reference))^(1/2)τ;    and the dominant noise source in the quadrature measurement of this    amplitude is due to the shot noise in the reference light 42, and    has the following number of photons impinging on each pixel:-   [5] dominant noise=((ε/hv)×P_(reference)×τ)^(1/2);    where ε is a detector efficiency scaling factor, P is the power for    each of the ultrasound tagged photons, hv is the per-photon energy    (with h as Plank's constant, and v as the frequency of the light),    and τ is the integrated pulse widths used in the measurement.

With a number of pixels N, the signal-to-noise ratio (SNR) scales withN^(1/2), since the total shot noise grows as N^(1/2), whereas the totalsignal grows as N, so that:

-   [6] SNR_(N pixels)=(N×(ε/hv)×τ×P_(signal))^(1/2), which shows that    the SNR improves with increasing number of pixels in the lock-in    camera 28. Thus, the Poisson nature of photon shot noise statistics    is being utilized to determine the fundamental signal to noise    ratio.

It should be appreciated that although the UOT system 10 has beendescribed as using a 4-bin quadrature detection scheme to reconstructthe amplitude of the signal light 44 from the interference lightpatterns 48, and therefore, utilizes four bins 70 (and four opticalpulses) for each detector 68 (or pixel) of the lock-in camera 28 tostore the intensity values of the respective four interference patterns48 over the four different phases, the UOT system 10 may utilize lessthan four phases (e.g., three phases equal to 0, 2π/3, and 4π/3), or mayeven utilize two phases (e.g., 0 and π) to reconstruct the amplitude ofthe signal light 44 from the interference light patterns 48, andtherefore utilizes three bins 70 (and three optical pulses) or only twobins 70 (and only two optical pulses) for each detector 68 (or pixel) tostore the intensity values of the respective interference patterns 48over the phases. It should further be appreciated that although thephases of the 4-bin quadrature scheme, as well as the three-bin andtwo-bin detection schemes, have been described as being equally spaced,the phases used in any of these detection schemes can be unequallyspaced. For example, for the three-bin detection scheme, the phases canbe selected to be 0, π, and 4π/3, or for a two-bin detection scheme, thephases can be selected to be 0 and 4π/3.

In the case of a two-bin detection scheme, rather than obtaining aquadrature amplitude from each pixel 68, the power of the signal light44 can be computed as the absolute difference between the two intensityvalues stored in the two bins 70 for each pixel 68 and then averaged inaccordance with the following equation:

-   [7] P_(signal)∝Avg(Σk|Value_(1,k)−Value_(2,k)|) across all k=1, 2, .    . . N speckles or pixels, where Value₁ is the intensity value in the    first bin 70 of the respective pixel 68, and Value₂ is the intensity    value in the second bin 70 of the respective pixel 68. Thus, it can    be appreciated that the intention of the two-bin measurement is to    arrive at a precise estimate of power of the signal light P_(signal)    up to the aforementioned scaling relationship defined by the    strength of the reference light P_(reference), by removing the terms    that are constant between the two measurements of the lock-in camera    28, and removing the unknown speckle-specific phases, and instead    extracting only the amplitude of the cosine term. In the context of    the UOT, just as with the quadrature detection scheme described    above, the two-bin detection scheme serves as a measurement of light    absorption at a single spatial voxel within the tissue.

However, the two-bin detection scheme represents a simplification thatleads to only a small constant decrease factor in the signal to noiseratio. The dominant signal source contributing to detection has thefollowing number of photons impinging on one pixel:

-   [4] dominant    signal=(ε/hv)×B×τ/2×|cos(0+φ)−cos(π+φ)|_(average over φ in [0,2π]);    and the dominant noise source in the two-bin measurement of this    amplitude is due to the shot noise in the reference light 42, and    has the following number of photons impinging on one pixel:-   [5] dominant noise=((ετ/hv)^(1/2)×(2/π)B/A^(1/2),    where A and B are constants in the equation A+B(cosφ), B is    proportional to the number of tagged photons per detector 68, ε is a    detector efficiency scaling factor, hv is the per-photon energy    (with h as Plank's constant, and v as the frequency of the light),    and τ is the integrated pulse widths used in the measurement, and φ    is a random, pixel-specific speckle phase.

With a number of pixels N, the signal-to-noise ratio (SNR) scales withN^(1/2), since the total shot noise grows as N^(1/2), whereas the totalsignal grows as N, so that:

-   [6] SNR_(N pixels)=(N×(ε τ/hv)^(1/2)×(2/π)×B/A^(1/2), which shows    that, just as in the quadrature detection scheme, the SNR improves    with increasing number of pixels in the lock-in camera 2828, and the    Poisson nature of photon shot noise statistics is being utilized to    determine the fundamental signal to noise ratio.

Notably, the use of a two-bin detection scheme, rather than the four-binquadrature scheme, provides the advantage that only two optical pulses,as opposed to four optical pulses, needs to be generated, therebyshortening the time period needed to take a measurement of the targetvoxel 14, and thus, alleviating the speckle decorrelation timelimitation.

In an optional embodiment, a digital optical phase conjugation (DOPC)technique can be used to boost the sensitivity of the pulsed UOTdetection. DOPC can be performed in the context of schemes that rely ontime reversal based optical phase conjugation using “guidestars”localized in three dimensions, for instance, using schemes, such as TimeReversal of Ultrasound-Encoded Light (TRUE) (see, e.g., Xu X, Liu H.,Wang L V, “Time-Reversed Ultrasonically Encoded Optical Focusing intoScattering Media,” Nature Photonics, Vol. 5, No. 3, pp. 154-157 (Mar. 1,2011); Wang Y M, Judkewitz B, DiMarzio C A, Yang C., “Deep-Tissue FocalFluorescence Imaging with Digitally Time-Reversed Ultrasound-EncodedLight,” Nature Communications, Vol. 3, Article 928 (Jun. 16, 2012);Horstmeyer R., Ruan H, Yang C, “Guidestar-Assisted Wavefront-ShapingMethods for Focusing Light into Biological Tissue,” Nature Photonics,Vol. 9, No. 9, pp. 563-571 (Sep. 1, 2015).

These methods are used to focus light to a guide-star-defined point deepinside a scattering medium, by measuring the wavefront emanating fromthe guidestar and digitally time-reversing (e.g., phase conjugating)light in order to cause the light to “play back” its path through thescattering medium and come to focus at the guidestar position. In thecontext of UOT, the guidestar is the focal point of an ultrasound beam.In these methods, the phase of a tagged light field originating from agiven three-dimensional guidestar voxel in the brain is measured usingdemodulation and quadrature detection, and then an approximatephase-conjugate, i.e., approximate time-reverse light field, possiblyamplified in intensity, is “played back” to focus light to thethree-dimensional guidestar location despite the effects of strong oreven diffusive scattering in the tissue.

In the context of the UOT system 10 described herein, the phase of thewavefront of the signal light 44 originating from the target voxel 14(i.e., the guidestar) is measured using the pulsed UOT detection schemedescribed above, as illustrated in FIG. 9 a, with the exception that, inaddition to extracting the power of the wavefront of the P_(signal), theunknown phase φ_(unknown) of the wavefront of the signal light 44 isextracted using the known principles of quadrature detection. Asillustrated in FIG. 9 b, the wavefront of the signal light 44 is thenamplified and retransmitted back in the opposite direction and focusedonto the target voxel 14 (i.e., the guidestar), where it is tagged bythe ultrasound 32 and may then be detected by the same or anotherlock-in camera so as to perform the process iteratively. Theretransmission should be timed such that the light through the voxel iscoincident in time with an ultrasound pulse passing through the voxel.

Referring to FIG. 10 a, the phase of the signal light 44 extracted inthe pulsed UOT detection scheme may be used in conjunction with aspatial light modulator (SLM) array 78 that is co-registered (e.g.,pixel-by-pixel) with the lock-in camera 28 to perform optical phaseconjugation of the signal light 44 (see, e.g., Laforest T, Verdant A,Dupret A, Gigan S., Ramaz F, Tessier G, “Co-Integration of a Smart CMOSImage Sensor and a Spatial Light Modulator for Real-Time Optical PhaseModulation,” Proc. Of SPIE-IS&T, Vol. 2014, 9022:90220N-1 (March 2014).

The SLM array 78 may include any of a number of different amplitudeand/or phase modulator structures, such as liquid crystals,re-positionable microelectromechanical systems (MEMS) mirrors,ferroelectrics, digital micro-mirror device pixels, among others. In oneembodiment, the SLM array 78 may be semi-transparent (e.g., a liquidcrystal modulator backed by a partial reflector), and can be insertedinto the light path between the entry of the reference light 42 and thelock-in camera 28. The SLM array 78 may be built directly on top of thelock-in camera 28 to create a phase conjugation array, with thisarrangement being similar to the pulsed UOT detection scheme describedabove.

Referring to FIG. 10 b, post-detection, each pixel 68 of the lock-incamera 28 will send the conjugate phase information to the SLM array 78(conjugate phase information being the negative of the detected phase).Each pixel may have internal electronics (e.g., transistors) thatcompute and sends the desired phase adjustment as an electrical voltageto adjust the corresponding pixel phase or amplitude of the SLM array78. In a phase-only optical phase conjugation scheme, each pixel of thephase conjugation array will simply “actuate” the conjugate phase, suchthat light reflected from the pixel will accrue the phase.Amplitude-only phase conjugation can alternatively be performed byreflecting or not reflecting the input light based on the conjugatephase information. In aggregate, the same reference light 42 used in thedetection process (while blocking the sample light 40) or lightprecisely aligned with the reference light 42 will reflect off the phaseconjugation array to create a phase conjugate light field 80 that willfocus back to the target voxel 14.

The improvement in contrast of this return light 80 (i.e., the phaseconjugate light field) to the target voxel 14 is given by: ContrastA=α*((N−1)/M+1), wherein N is the number of input optical modes (or thenumber of photons if less than the number of input optical modes), whichis approximately equal to the number of pixels on the phase conjugationarray); M is the number of target optical modes in the target voxel 14,and a equals 1 when a full phase and amplitude conjugation is performed,and is some value smaller than 1 when a phase only, amplitude only,and/or coarse grain phase conjugation is performed. The term “coarsegrain,” in this context, means that the phase playback at each pixel cantake only a finite number of possible values.

The phase conjugation process can be iterated many times, each timetaking the light field, resulting from the last step, and phaseconjugating that scattered light field. The contrast improvement can beexpected to grow as (contrast A)^(K), where K is the number ofiterations. Thus, the number of photons traveling through the targetvoxel 14 can be exponentially amplified, thereby improving the effectivemodulation depth of the UOT (i.e., the fraction of the ultrasound taggedphotons reaching the detector). The addition of phase conjugation to thepulsed UOT system 14 could be used to increase the number of collectedtagged photons, increase modulation depth, or decrease ultrasoundintensity or duty cycle requirements.

Performance estimates for the UOT system 10 described herein in thedetection of a blood-oxygen-level dependent signal in the brain throughthe skull as a function of the number of pixels in the lock-in camera 28used (in this case, 10 million pixels or higher) indicate that neuralactivity dependent changes in the blood-oxygen-level dependent signalcould be detected at hundreds to thousands of voxels per 100 millisecondtemporal sample. In this calculation, the use of a 2 MHz ultrasound, andthus a spatial resolution on the order of ½ millimeter, is assumed,exceeding the spatial resolution of traditional blood-oxygen-leveldependent signal measurements, like functional MRI (fMRI), and vastlyexceeding the many millimeter to multiple centimeter-scale spatialresolution of diffuse optical tomography, including time-gated forms ofdiffuse optical tomography. In this calculation, it is further assumedthat millions of tagged photons must be collected from the target voxel14 per temporal sample in order to measure naturally occurringblood-oxygen-level dependent signals functional changes in the humanbrain, which are on the order of small fractions of a percent, whileovercoming shot noise fluctuations in the number of detected taggedphotons.

In one embodiment, the processor 30 utilizes blood-oxygen-leveldependent signals detected by the lock-in camera 28 to determine theneural activity in the brain; that is, blood-oxygen-level dependentsignals provide a sense of the level of deoxygenated and/or oxygenatedhemoglobin concentration or relative abundance in the target voxel 14 inthe brain, and given the known coupling between cerebral hemodynamicsand neuronal activity, the processor 30 can thereby determine the extentof neuronal activity in that target voxel 14. In another embodiment, theUOT system 10 detects blood-oxygen-level dependent signals over multiplewavelengths of the sample light, in which case, the processor 30 maydetermine and compare the optical absorption characteristics of thetarget voxel 14 of blood-oxygen-level dependent signals over thedifferent wavelengths of sample light in order to determine the level ofdeoxygenated and/or oxygenated hemoglobin concentration or relativeabundance present in the target voxel 14 according to known principlesof functional infrared spectroscopy, for instance by solving twoequations in two unknowns relating the measured absorption at twowavelengths to the level of deoxygenated and/or oxygenated hemoglobinconcentration or relative abundance in the blood, or alternativelyseveral equations in several unknowns representing absorption at severalwavelengths in order to determine the concentrations of severalmolecular species in the target voxel 14.

In one particularly advantageous embodiment, instead of detectingblood-oxygen-level dependent signals, the processor 30 may detect fastersignals of neuronal activity, such as in the brain, to determine theextent of neuronal activity in the target voxel 14. Neuronal activitygenerates fast changes in optical properties, called “fast signals,”which have a latency of about 10-100 milliseconds and are much fasterthan the metabolic (approximately 100-1000 milliseconds) and hemodynamic(hundreds of milliseconds to seconds) evoked responses (seeFranceschini, M A and Boas, D A, “Noninvasive Measurement of NeuronalActivity with Near-Infrared Optical Imaging,” Neuroimage, Vol. 21, No.1, pp. 372-386 (January 2004)). Additionally, is believed that brainmatter (e.g., neurons and the extracellular matrix around neurons)hydrates and dehydrates as neurons fire (due to ion transport in and outof the neurons), which could be measured via determining the absorptioncharacteristics of water in the target voxel 14. In this case, it ispreferred that the target voxel 14 be minimized as much as possible byselecting the appropriate ultrasound frequency (e.g., two to six timesthe size of a neuron, approximately 100 micrometers) in order tomaximize sensitivity to highly localized changes in fast indicators ofneural activity. As illustrated in FIG. 11, the optical absorptioncoefficient of water is relatively high for wavelengths of light in therange of 950 nm-1080 nm. Thus, for maximum sensitivity to changes inoptical absorption of tissue due to changes in the level of waterconcentration or relative water concentration in the brain matter, it ispreferred the wavelength of the sample light be in the range of 950nm-1080 nm.

Regardless of the nature of the detected signal andphysiologically-dependent optical parameter, the processor 30 mayoptionally use a computational model of light propagation in the tissue,and deconvolution or inverse problem optimization methods/algorithms, toimprove the spatial resolution of the resulting measurement. Empiricalmeasurements of a sample may be compared to those predicted by a modelof the spatial layout of absorbers of the sample incorporating aneffective point spread function of detection, such that the model may beimproved to obtain an optimal match between the model predictions andthe observed signals from the sample (see Powell S., Srridge S R, LeungT S, “Gradient-Based Quantitative Image Reconstruction inUltrasound-Modulated Optical Tomography: First Harmonic Measurement Typein a Linearized Diffusion Formulation,” IEEE Transactions on MedicalImaging, Vol. 35, No. 2, pp. 456-467 (February 2016).

Although the UOT system 10 has been described herein as acquiring onlyone measurement of the target voxel 14, it should be appreciated thatthe UOT system 10 may acquire multiple measurements of the target voxel14 over time that yields a time trace indicative of time varyingphysiologically depending optical properties in the target voxel 14,such as time-varying optical absorption in the target voxel 14 due tofunctional changes in the brain. Optionally, two time traces of thetarget voxel 14 can be acquired, one time trace being generated with theultrasound 32 turned on at regular intervals in the same mannerdescribed above, and another time trace generated with the ultrasound 32turned off at regular intervals. For example, a measurement of thetarget voxel 14 may be acquired when the ultrasound 32 turned on tocreate a first data point on the first time trace; a measurement of thetarget voxel 14 may be acquired when the ultrasound 32 turned off tocreate a first data point on the second time trace; a measurement of thetarget voxel 14 may be acquired when the ultrasound 32 turned on tocreate a second data point on the first time trace; a measurement of thetarget voxel 14 may be acquired when the ultrasound 32 turned off tocreate a second data point on the second time trace; and so forth. Thesecond time trace may provide a baseline null signal measurement trace,which is useful for tracking secondary variations distinct from thefirst time trace's signal variations due to the ultrasound 32.

Referring now to FIG. 12, the physical implementation of the UOT system10 will be described. As there shown, the UOT system 10 includes awearable unit 90 that is configured for being applied to the patient 18,and in this case, worn on the head of the patient 18, and an auxiliaryhead-worn or not head-worn unit 92 coupled to the wearable unit 90 via awired connection 94 (e.g., electrical wires). Alternatively, the UOTsystem 10 may use a non-wired connection (e.g., wireless radio frequency(RF) signals) for providing power to or communicating between componentsof the respective wearable unit 90 and auxiliary unit 92.

In the illustrated embodiment, the wearable unit 90 includes a supportstructure 94 that either contains or carries the transducer arrangement34 of the acoustic assembly 20 (shown in FIG. 2), the interferometer 22,and the lock-in camera 28. The wearable unit 90 may also include anoutput port 98 a from which the ultrasound 32 generated by the acousticassembly 20 (shown in FIG. 1) is emitted, an output port 98 b from whichthe sample light 38 generated by the interferometer 22 (shown in FIG. 1)is emitted, and an input port 98 c into which the sample light pattern47 comprising the tagged signal light and untagged background light areinput into the interferometer 22. It should be appreciated that althoughthe input port 98 c is illustrated in close proximity to the input ports98 a, 98 b, the proximity between the input port 98 c and the outputports 98 a, 98 b may be any suitable distance. The support structure 94may be shaped, e.g., have a banana, headband or hat shape, such that theports 98 are in close contact with the outer skin of the body part, andin this case, the scalp of the head 18 of the patient 18. An indexmatching fluid maybe used to reduce reflection of the light generated bythe light source 30 of the interferometer 22 from the outer skin, and anultrasound transmitting adhesive or acoustic coupling material can beused to facilitate conduction of the ultrasound 32 into the body part.An adhesive or belt (not shown) can be used to secure the supportstructure 94 to the body part.

The auxiliary unit 92 includes a housing 96 that contains the controller24 and the processor 30 (shown in FIG. 1). The auxiliary unit 92 mayadditionally include a power supply (which if head-worn, may take theform of a rechargeable or non-chargeable battery), a control panel withinput/output functions, a display, and memory.

The auxiliary unit 92 may further include the signal generator 36 of theacoustic assembly 20, as well as any drive circuitry used to operate theinterferometer 22.

The interferometer 22 and lock-in camera 28 are preferably mechanicallyand electrically isolated from the acoustic assembly 20, such that theemission of the ultrasound 32 by the acoustic assembly 20, as well asthe generation of RF and other electronic signals by the acousticassembly 20 minimally affects the detection of the optical signals bythe interferometer 22 and generation of data by the lock-in camera 28.The wearable unit 90 may include shielding (not shown) to preventelectrical interference and appropriate materials that attenuate thepropagation of acoustic waves through the support structure 94.

Having described the arrangement of function of the UOT system 10, onemethod of operating the UOT system on a patient will now be described.In this method, ultrasound 32 is delivered into the target voxel 14 inthe anatomical structure 16, and sample light 40 is delivered into theanatomical structure 16, wherein a portion 40 a of the sample light 40passing through the target voxel 14 is scattered by the anatomicalstructure 16 as the signal light 44, and another portion 40 b of thesample light 40 not passing through the target voxel 14 is scattered bythe anatomical structure 16 as background light 46 that combines withthe signal light 44 to create the sample light pattern 47. Asexemplified above, the anatomical structure 16 may be an intact headcomprising the scalp, skull, and brain matter. Due to the highresolution of the UOT system 10, the target voxel 14 may be smaller thanone mm³.

The reference light 42 is combined with the sample light pattern 47 togenerate an interference light pattern 48 (e.g., in a homodyne manner),and in this method, a speckle light pattern. The ultrasound 32 andsample light 40 are pulsed in synchrony, such that only the signal light44 is shifted (i.e., tagged) by the ultrasound 32. That is, as describedabove, each pulse of the sample light 40 will pass through the targetvoxel 14 only as the ultrasound 32 passes through the target voxel 14,such that no portion of the background light 46 will be tagged by theultrasound 32. The interference light pattern 48 is sequentiallymodulated to generate a plurality of different interference lightpatterns 48. The spatial components of any particular interference lightpattern 48 can then be simultaneously detected, and a plurality ofvalues can be stored in the respective bins 70 (either in bins 70 a, inbins 70 b, in bins 70 c, or bins 70 d) of the detectors 68. The valuesare representative of the spatial component for the respectiveinterference light pattern 48. The physiologically-dependent opticalparameter of the target voxel 14 is then determined based on the spatialcomponent values stored in the bins 70. Due to the high speed of thelock-in camera 28, the spatial components for any particularinterference light pattern 48 may be simultaneously detected and storedin the respective bins 70 very quickly. For example, in one embodiment,the spatial components for any particular interference light pattern 48may be simultaneously detected, and the resulting spatial componentvalues for all the interference light patterns 48 are stored in therespective bins 70 within 1 millisecond. In another embodiment, thespatial components for any particular interference light pattern 48 maybe simultaneously detected, and the resulting spatial component valuesfor all the interference light patterns 48 are stored in the respectivebins 70 within 1 microsecond to 1 millisecond.

Referring to FIG. 13, one particular method 100 performed by the UOTsystem 10 to non-invasively image the target voxel 14 in anatomicalstructure 16 will now be described. This particular method 100implements the pulsing sequence of FIG. 5.

The controller 24 operates the acoustic assembly 20 to generate anddeliver a pulse of ultrasound 32 having a frequency f_(us) (initially,ultrasound pulse U_(a) illustrated in FIG. 5) into the anatomicalstructure 16, e.g., by sending a control signal to the signal generator36 to pulse an electrical signal on and off (step 102). The controller24 sets the phase difference between the sample light 40 and thereference light 42 to one of the four pre-defined values (0, π/2, π, and3π/2) by sending a control signal to the phase shifter 54 of theinterferometer 22 (step 104). This pre-defined phase difference valuemay be first set to 0. Next, the controller 24 operates theinterferometer 22 to generate and emit a pulse of source light 38 havinga frequency f, e.g., by sending a control signal to the drive circuit topulse the light source 50 on and off (step 106). The interferometer 22(e.g., via the beam splitter 52) splits the pulse of source light 38into a pulse of sample light 40 (initially, the sample light pulse L_(a)illustrated in FIG. 5) and a pulse of reference light 42 (step 108).

The wavelength (and thus, the frequency f) of the source light 38 may beselected based on the physiologically-dependent optical parameter to beultimately determined. For example, if the physiologically-dependentoptical parameter is the level of deoxygenated and/or oxygenatedhemoglobin concentration or relative abundance, the wavelength of thesource light 38 may be in the range of 605 nanometers to 950 nanometers,whereas if the physiologically-dependent optical parameter to bedetermined is a water absorption level (level of water concentration orrelative water concentration), the wavelength of the source light 38 maybe in the range of 950-1080 nanometers.

Next, prior to the pulse of sample light 40 entering the anatomicalstructure 16, the controller 24 operates the interferometer 22 tofrequency shift the pulse of sample light 40 by the ultrasound frequencyf_(us), e.g., by sending a control signal to the frequency shifter 56,resulting in the pulse of sample light 40 having a frequency f−f_(us)(step 110). The frequency-shifted pulse of sample light 40 is thendelivered into and diffusively scattered within the anatomical structure16 (step 112). As the pulse of frequency shifted sample light 40scatters diffusively through the anatomical structure 16, a portion willpass through the target voxel 14 and be frequency shifted (i.e., tagged)back to its original frequency f by the pulse of ultrasound 32 passingthrough the target voxel 14, resulting in a pulse of scattered signallight 44 having the same frequency f (step 114); and remaining portionwill not pass through the target voxel 14, and thus will not befrequency shifted by the pulse of ultrasound 32, resulting in a pulse ofscattered background light 46 having a frequency f−f_(us) (the samefrequency as the frequency shifted sample light 40 prior to entering theanatomical structure 16) (step 116).

Next, the interferometer 22 then combines (e.g., via the light combiner58) the pulse of reference light 42 with the pulses of sample lightpattern 47 to generate a pulse of an interference light pattern 48(initially, the interference light pattern pulse I_(a) illustrated inFIG. 5) (step 118). Then, under control of the controller 24, all of thedetectors 68 (FIG. 4) of the lock-in camera 28 simultaneously detectrespective spatial components of the interference light pattern 48(i.e., speckle grains in the case where the interference light patternincludes a speckle light pattern) (step 120), and values (e.g., powervalues) representative of the spatial components of the interferencelight pattern 48 are stored in bins 70 (initially, the first bins 70 aof the corresponding detectors 68) (step 122).

At this point, only one quadrature measurement has been taken. If theinterferometer 22 has not been set to all four of the phases (step 124),the controller 24 then repeats steps 102-122 to take the next quadraturemeasurement. That is, the next pulse of ultrasound 32 (e.g., ultrasoundpulse U_(b) illustrated in FIG. 5) is generated and emitted into theanatomical structure 16 (step 102); the phase difference between thesample light 40 and the reference light 42 is set to the nextpre-defined value (e.g., 7/2) (step 104); the next pulse of source light38 is generated (step 106) and split into the next pulse of sample light40 (e.g., sample light pulse L_(b) illustrated in FIG. 5) and a pulse ofreference light 42 (step 108); the next pulse of sample light 40 isfrequency shifted by the ultrasound frequency f_(us) (step 110); thefrequency shifted pulse of sample light 40 is delivered and diffusivelyscattered within the anatomical structure 16 (step 112); a portion ofthe scattered pulse of sample light 40 passing through the target voxel14 is frequency shifted (tagged) by the pulse of ultrasound 32 passingthrough the target voxel 14, thereby generating the next pulse ofscattered signal light 44 (step 114); the remaining portion of thescattered pulse of sample light 40 not passing through the target voxel14 is not frequency shifted (not tagged) by the pulse of ultrasound 32passing through the target voxel 14, thereby generating the next pulseof scattered background light 46 (step 116); the next pulses ofreference light 42 and sample light pattern 47 are combined into thepulse of the next interference light pattern 48 (e.g., the interferencelight pattern pulse I_(b) illustrated in FIG. 5) (step 118); the spatialcomponents of the next pulse of the interference light pattern 48 aredetected (step 120); and the resulting spatial component values arestored in the bins 70 (e.g., the second bins 70 b of the correspondingdetectors 68) (step 122).

Thus, it can be appreciated that steps 102-122 will be repeated to takethe remaining quadrature measurements to generate and detect the pulsesof the remaining interference light patterns (e.g., the third and fourthinterference light pattern pulses I_(c), I_(d) illustrated in FIG. 5)for the remaining phase settings (e.g., π and 3π/2) and ultimate storageof the spatial component values in the bins 70 (e.g., the third andfourth bins 70 c, 70 d of the corresponding detectors 68).

After all four quadrature measurements have been taken, the controller24 recalls the spatial component values of the detected interferencelight pattern pulses 48 from the bins 70 of the lock-in camera 28 andtransfers these values to the processor 30 (step 126). The processor 30reconstructs the amplitude of the signal light 44 from the fourinterference light patterns 48 based on these spatial component values(e.g., by using the quadrature equation [2]) (step 128). Steps 102-128can be iterated to repeatedly acquire data measurements of the targetvoxel 14, and if a sufficient number of data measurements have beenacquired (step 130), the processor 30 may then determine thephysiologically-dependent optical parameter (e.g., level of deoxygenatedand/or oxygenated hemoglobin concentration or relative abundance orlevel of water concentration or relative water concentration) of thetarget voxel 14 based on the data measurements (step 132). In the casewhere the target voxel 14 is brain matter, the processor 30 may furtherdetermine the level of neural activity within the target voxel 14 basedon the determined physiologically-dependent optical parameter (step134).

For example, if the physiologically-dependent optical parameter is thelevel of deoxygenated and/or oxygenated hemoglobin concentration orrelative abundance, and if the amplitude of the signal light 44 isrelatively low (or high), indicating high absorption of light by bloodin the target voxel 14, it can be assumed that there is a relativelyhigh (or low) hemodynamic response (depending on the light wavelengthused) through the target voxel 14, and thus, a substantial amount ofneural activity in the target voxel 14. In contrast, if the amplitude ofthe signal light 44 is relatively high (or low), indicating lowabsorption of light by blood in the target voxel 14, it can be assumedthat there is a relatively low hemodynamic response (depending on thewavelength) through the target voxel 14, and thus, comparatively littleneural activity in the target voxel 14.

If the physiologically-dependent optical parameter is the level of waterconcentration or relative water concentration, and if the amplitude ofthe signal light 44 greatly varies over a short period of time,indicating a fast signal of neural activity in the brain tissue, it canbe assumed that there is a substantial amount of neural activity in thetarget voxel 14. In contrast, if the amplitude of the signal light 44varies very little over a short period of time, indicating that there isno fast signal of neural activity in the brain matter, it can be assumedthat there is very little or no neural activity in the target voxel 14.

Referring to FIG. 14, another particular method 100′ performed by theUOT system 10 to non-invasively image the target voxel 14 in theanatomical structure 16 will now be described. This particular method100′ implements the pulsing sequence of FIG. 7.

The method 100′ is similar to the method 100 illustrated in FIG. 13,with the exception that, instead of a one-to-one correspondence betweenthe sample light pulse and the ultrasound pulse, multiple sample pulses(in this case, four) are delivered to the biological tissue for everyultrasound pulse delivered to the biological tissue. Thus, after aquadrature measurement, the process returns to step 104 (instead of step102) to take the next quadrature measurement.

That is, during the delivery of current pulse of ultrasound 32 (e.g.,ultrasound pulse U illustrated in FIG. 5), the phase difference betweenthe sample light 40 and the reference light 42 is set to the nextpre-defined value (e.g., 7/2) (step 104); the next pulse of source light38 is generated (step 106) and split into the next pulse of sample light40 (e.g., sample light pulse L_(b) illustrated in FIG. 5) and a pulse ofreference light 42 (step 108); the next pulse of sample light 40 isfrequency shifted by the ultrasound frequency f_(us) (step 110); thefrequency shifted pulse of sample light 40 is delivered and diffusivelyscattered within the anatomical structure 16 (step 112); a portion ofthe scattered pulse of sample light 40 passing through the target voxel14 is frequency shifted (tagged) by the pulse of ultrasound 32 passingthrough the target voxel 14, thereby generating the next pulse ofscattered signal light 44 (step 114); the remaining portion of thescattered pulse of sample light 40 not passing through the target voxel14 is not frequency shifted (not tagged) by the pulse of ultrasound 32passing through the target voxel 14, thereby generating the next pulseof scattered background light 46 (step 116); the next pulses ofreference light 42 and sample light pattern 47 are combined into thepulse of the next interference light pattern 48 (e.g., the interferencelight pattern pulse I_(b) illustrated in FIG. 5) (step 118); the spatialcomponents of the next pulse of the interference light pattern 48 aredetected (step 120); and the resulting spatial component values arestored in the bins 70 (e.g., the second bins 70 of the correspondingdetectors 68) (step 122).

After all four quadrature measurements have been taken at steps 102-124,as in the manner described above with respect to the method 100 of FIG.13, the spatial component values of the detected interference lightpattern pulses 48 are recalled from the bins 70 of the lock-in camera 28(step 126); and the amplitude of the signal light 44 is reconstructedfrom the four interference light patterns 48 based on these spatialcomponent values (step 128). Steps 102-128 can be iterated to repeatedlyacquire data measurements of the target voxel 14, and if a sufficientnumber of data measurements have been acquired (step 130), thephysiologically-dependent optical parameter (e.g., the level ofdeoxygenated and/or oxygenated hemoglobin concentration or relativeabundance or level of water concentration or relative waterconcentration) of the target voxel 14 is determined based on the datameasurements (step 132), and the level of neural activity within thetarget voxel 14 is determined based on the determinedphysiologically-dependent optical parameter (step 134).

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

1. An ultrasound modulated optical tomography (UOT) system, comprising:an acoustic assembly configured for delivering ultrasound into a targetvoxel in an anatomical structure; an interferometer configured fordelivering sample light into the anatomical structure, whereby a portionof the sample light passing through the target voxel is scattered by theanatomical structure as signal light, and another portion of the samplelight not passing through the target voxel is scattered by theanatomical structure as background light that combines with the signallight to create a sample light pattern, the interferometer furtherconfigured for combining reference light with the sample light patternto generate an interference light pattern; a controller configured foroperating the acoustic assembly and the interferometer to pulse theultrasound and the sample light in synchrony, such that only the signallight is frequency shifted by the ultrasound, wherein a duration of eachpulse of the ultrasound delivered into the target voxel is within adecorrelation time of the target voxel, and wherein multiple pulses ofthe sample light are sequentially delivered into the anatomicalstructure for each pulse of the ultrasound delivered into the targetvoxel, the controller further configured for operating theinterferometer to sequentially modulate the interference light patternto generate a plurality of different interference light patterns; atleast one detector configured for detecting the different interferencelight patterns; and a processor configured for determining aphysiologically-dependent optical parameter of the target voxel based onthe detected interference light patterns.
 2. The UOT system of claim 1,wherein the at least one detector comprises an array of detectorsconfigured for respectively detecting spatial components of eachdifferent interference light pattern, wherein the processor isconfigured for determining the physiologically-dependent opticalparameter of the target voxel based on the detected spatial components.3. The UOT system of claim 2, wherein each detector, in response toexternal trigger signals generated by the controller, is configured forrespectively storing a plurality of values in a plurality of datastoring bins representative of the respective spatial components of theinterference light patterns, wherein the processor is configured fordetermining the physiologically-dependent optical parameter of thetarget voxel based on the plurality of values stored in the data storingbins of each detector.
 4. The UOT system of claim 1, wherein thephysiologically-dependent optical parameter is the level of deoxygenatedand/or oxygenated hemoglobin concentration or relative abundance.
 5. TheUOT system of claim 1, wherein the physiologically-dependent opticalparameter is the level of water concentration or relative waterconcentration of brain matter.
 6. The UOT system of claim 5, wherein thewavelength of the sample light is in the range of 950-1080 nanometers.7. The UOT system of claim 1, wherein the anatomical structure is ahead, the system further comprising a wearable unit configured for beingaffixed to the head, the wearable unit comprising the acoustic assemblyand the interferometer.
 8. The UOT system of claim 1, wherein the targetvoxel comprises brain matter, and the processor is configured fordetermining whether or not neural activity exists within the targetvoxel based on the determined physiologically-dependent opticalparameter.
 9. The UOT system of claim 1, wherein the interferometer isconfigured for sequentially modulating the interference light pattern byphase modulating the interference light pattern.
 10. The UOT system ofclaim 9, wherein the interferometer is configured for phase modulatingthe interference light pattern by respectively setting different phasesbetween sequential pulses of the sample light and the reference light.11. The UOT system of claim 10, wherein the different phases are 0, π/2,π, and 3π/2.
 12. The UOT system of claim 1, wherein the pulses of thesample light are identical.
 13. The UOT system of claim 12, wherein theinterferometer comprises: at least one 1×N fiber splitter configured forsplitting a single optical pulse into an N number of identical lightpulses; an N number of optical fibers configured for propagating the Nnumber of identical light pulses, wherein the N number of optical fibersare of different lengths, such that the N number of identical lightpulses are temporally spaced apart; at least one N×1 fiber couplerconfigured for temporally combining the N number of identical lightpulses; and a single optical fiber for propagating the N number oftemporally combined identical light pulses as the sample light pulses.14. The UOT system of claim 1, wherein the interferometer is configuredfor combining the reference light with the signal light using a homodynetechnique.
 15. The UOT system of claim 14, wherein the interferometer isfurther configured for frequency shifting the sample light by thefrequency of the ultrasound, such that the reference light is combinedwith the signal light using the homodyne technique.
 16. The UOT systemof claim 1, wherein the interferometer comprises a light sourceconfigured for generating source light, a beam splitter configured forsplitting the source light into the sample light and the referencelight, and a light combiner configured for combining the reference lightwith the signal light and the background light to generate theinterference light pattern.
 17. The UOT system of claim 16, wherein theinterferometer further comprises an optical phase shifter configured forsetting a phase difference between the sample light and the referencelight to phase modulate the interference light pattern.
 18. The UOTsystem of claim 3, wherein each of the interference light patternscomprises a speckle light pattern.
 19. The UOT system of claim 18,wherein the spatial components comprise speckle grains of the specklelight pattern.
 20. The UOT system of claim 3, wherein the array ofdetectors is configured for respectively detecting spatial components ofeach different interference light pattern, and storing the plurality ofvalues for all of the interference patterns in the plurality of datastoring bins within 10 milliseconds.
 21. The UOT system of claim 3,wherein the array of detectors is configured for respectively detectingspatial components of each different interference light pattern, andstoring the plurality of values for all of the interference patterns inthe plurality of data storing bins within 1 microsecond to 1millisecond.
 22. The UOT system of claim 3, wherein the processor isconfigured for reconstructing the amplitude of the signal light usingthe plurality of values stored in each of the data storing bins, anddetermining the physiologically-dependent optical parameter of thetarget voxel based on the reconstructed amplitude of the signal light.23. The UOT system of claim 3, wherein each value respectively stored ineach of the data storing bins is an intensity of the spatial componentof the respective interference light pattern, the processor isconfigured for using the plurality of values stored in each of the datastoring bins to extract a product of the amplitude of the signal lightand a known amplitude of the reference light, and determining theamplitude of the signal light from the extracted product.
 24. The UOTsystem of claim 1, wherein the target voxel is less than one mm³, andthe acoustic assembly is configured for focusing the ultrasound onto thetarget voxel.
 25. The UOT system of claim 3, further comprising alock-in camera that includes the array of detectors and thecorresponding data storing bins.
 26. A method of performing pulsedultrasound modulated optical tomography (UOT), comprising: deliveringultrasound into a target voxel in an anatomical structure; deliveringsample light into the anatomical structure, whereby a portion of thesample light passing through the target voxel is scattered by theanatomical structure as signal light, and another portion of the samplelight not passing through the target voxel is scattered by theanatomical structure as background light that combines with the signallight to create a sample light pattern; pulsing the ultrasound and thesample light in synchrony, such that only the signal light is frequencyshifted by the ultrasound, wherein a duration of each pulse of theultrasound delivered into the target voxel is within a decorrelationtime of the target voxel, and wherein multiple pulses of the samplelight are sequentially delivered into the anatomical structure for eachpulse of the ultrasound delivered into the target voxel; combiningreference light with the sample light pattern to generate aninterference light pattern; sequentially modulating the interferencelight pattern to generate a plurality of different interference lightpatterns; detecting the different interference light patterns; anddetermining a physiologically-dependent optical parameter of the targetvoxel based on the detected interference light patterns.
 27. The methodof claim 26, wherein spatial components of each different interferencelight pattern are respectively detected, wherein thephysiologically-dependent optical parameter of the target voxel isdetermined based on the detected spatial components.
 28. The method ofclaim 27, further comprising, in response to external trigger signals,respectively storing a plurality of values in a plurality of datastoring bins representative of the respective spatial components of theinterference light patterns, wherein the physiologically-dependentoptical parameter of the target voxel is determined based on theplurality of values stored in the data storing bins of each detector.29. The method of claim 26, wherein the physiologically-dependentoptical parameter is the level of deoxygenated and/or oxygenatedhemoglobin concentration or relative abundance.
 30. The method of claim26, wherein the physiologically-dependent optical parameter is the levelof water concentration or relative water concentration of brain matter.31. The method of claim 30, wherein the wavelength of the sample lightis in the range of 950-1080 nanometers.
 32. The method of claim 26,wherein the biological tissue is brain matter.
 33. The method of claim32, further comprising determining whether or not neural activity existswithin the target voxel based on the determinedphysiologically-dependent optical parameter.
 34. The method of claim 26,wherein the interference light pattern is sequentially modulated byphase modulating the interference light pattern.
 35. The method of claim34, wherein the interference light pattern is phase modulated byrespectively setting different phases between sequential pulses of thesample light and the reference light.
 36. The method of claim 35,wherein the different phases are π, π/2, π, and 3π/2.
 37. The method ofclaim 26, wherein the pulses of the sample light are identical.
 38. Themethod of claim 26, wherein the reference light is combined with thesignal light using a homodyne technique.
 39. The method of claim 38,further comprising frequency shifting the sample light by the frequencyof the ultrasound, such that the reference light is combined with thesignal light using the homodyne technique.
 40. The method of claim 26,further comprising generating source light, and splitting the sourcelight into the sample light and the reference light.
 41. The method ofclaim 26, wherein each of the interference light patterns comprises aspeckle light pattern.
 42. The method of claim 41, wherein the spatialcomponents comprise speckle grains of the speckle light pattern.
 43. Themethod of claim 28, wherein the spatial components of each differentinterference light pattern are respectively detected and the pluralityof values for all of the interference patterns are stored in theplurality of data storing bins within 10 milliseconds.
 44. The method ofclaim 28, wherein the spatial components of each different interferencelight pattern are respectively detected and the plurality of values forall of the interference patterns are stored in the plurality of datastoring bins within 1 microsecond to 1 millisecond.
 45. The method ofclaim 28, further comprising reconstructing the amplitude of the signallight using the plurality of values stored in each of the data storingbins, wherein the physiologically-dependent optical parameter of thetarget voxel is determined based on the reconstructed amplitude of thesignal light.
 46. The method of claim 28, wherein each valuerespectively stored in each of the data storing bins is an intensity ofthe spatial component of the respective interference light pattern,wherein the plurality of values stored in each of the data storing binsare used to extract a product of the amplitude of the signal light and aknown amplitude of the reference light, and the amplitude of the signallight is determined from the extracted product.
 47. The method of claim26, wherein the target voxel is less than one mm³, and wherein theultrasound is focused on the target voxel.
 48. The UOT system of claim3, wherein the controller is configured for synchronizing operation ofthe acoustic assembly, the interferometer, and the array of detectors,such that the different interference light patterns are detected atspecific points on a cycle of the ultrasound.
 49. The method of claim28, further comprising synchronizing the delivery of the ultrasound, thedelivery of the sample light, and the detection of the differentinterference light patterns, such that the different interference lightpatterns are detected at specific points on a cycle of the ultrasound.50. The UOT system of claim 1, wherein the multiple pulses of the samplelight have the same wavelength.
 51. The method of claim 26, wherein themultiple pulses of the sample light have the same wavelength.